diff --git a/src/jolt/time/fmt.clj b/src/jolt/time/fmt.clj
index f685251..b8530c8 100644
--- a/src/jolt/time/fmt.clj
+++ b/src/jolt/time/fmt.clj
@@ -1,9 +1,13 @@
 (ns jolt.time.fmt
   "DateTimeFormatter + DateTimeFormatterBuilder. A pattern engine over the library
-  temporals; month/day display names are English by default, or locale-specific
-  through the core jolt.host/locale-name (strftime) primitive."
-  (:require [jolt.host :as host]
+  temporals; localized patterns and month/day display names come from the
+  bundled jolt.time.locale-data tables (measured from the reference JVM), with
+  the core jolt.host/locale-name (strftime) primitive as a last resort."
+  (:require [clojure.string :as str]
+            [jolt.host :as host]
             [jolt.time.impl :as impl :refer [civil-from-days days-from-civil]]
+            [jolt.time.locale-data :as ld]
+            [jolt.time.number-data :as nd]
             [jolt.time.util :as u]
             [jolt.time.local :as l]
             [jolt.time.year :as y]
@@ -19,13 +23,15 @@
 (def ^:private en-days ["Monday" "Tuesday" "Wednesday" "Thursday" "Friday" "Saturday" "Sunday"])
 
 (defn- month-name [locale mo full?]
-  (if (or (nil? locale) (= "en" locale))
-    (let [n (nth en-months (dec mo))] (if full? n (subs n 0 3)))
+  ;; bundled tables first: they are right everywhere; strftime depends on the OS
+  ;; having the locale installed and silently falls back to English when it does not
+  (if-let [names (ld/months locale full?)]
+    (nth names (dec mo))
     (or (host/locale-name locale (dec mo) 1 (if full? "%B" "%b"))
         (let [n (nth en-months (dec mo))] (if full? n (subs n 0 3))))))
 (defn- day-name [locale dow full?]
-  (if (or (nil? locale) (= "en" locale))
-    (let [n (nth en-days (dec dow))] (if full? n (subs n 0 3)))
+  (if-let [names (ld/days locale full?)]
+    (nth names (dec dow))
     (or (host/locale-name locale 0 (mod dow 7) (if full? "%A" "%a"))
         (let [n (nth en-days (dec dow))] (if full? n (subs n 0 3))))))
 
@@ -78,7 +84,9 @@
               (let [c (nth pattern i) k (run-len i c)]
                 (cond
                   (= c \') (let [[j o] (scan-quote pattern i n out)] (recur j o))
-                  (or (= c \y) (= c \Y)) (recur (+ i k) (str out (if (>= k 4) (u/pad4 y) (u/pad2 (mod y 100)))))
+                  ;; java.time year semantics: y is the full year, yy the reduced
+                  ;; two-digit form, yyy+ padded (CLDR localized patterns use bare y)
+                  (or (= c \y) (= c \Y)) (recur (+ i k) (str out (cond (= k 1) (str y) (= k 2) (u/pad2 (mod y 100)) :else (u/pad4 y))))
                   (= c \M) (recur (+ i k) (str out (cond (= k 1) (str mo) (= k 2) (u/pad2 mo) (= k 3) (month-name locale mo false) :else (month-name locale mo true))))
                   (= c \d) (recur (+ i k) (str out (if (= k 1) (str d) (u/pad2 d))))
                   (= c \E) (recur (+ i k) (str out (day-name locale dow (>= k 4))))
@@ -96,7 +104,10 @@
 
 ;; --- DateTimeFormatter -------------------------------------------------------
 (defn formatter [pattern locale] (impl/value :jolt.time/dt-formatter {:pattern pattern :locale (or locale "en")}))
-(defn- fmt-pattern [f] (impl/field f :pattern))
+(defn- fmt-pattern [f]
+  (or (impl/field f :pattern)
+      (let [[kind style] (impl/field f :style)]
+        (or (ld/pattern (impl/field f :locale) kind style) "yyyy-MM-dd HH:mm:ss"))))
 (defn- fmt-locale [f] (impl/field f :locale))
 (defn fmt? [f] (= :jolt.time/dt-formatter (impl/type-of f)))
 
@@ -118,7 +129,10 @@
 
 (__register-class-methods! :jolt.time/dt-formatter
   {"format" (fn [self v] (format-pattern (fmt-pattern self) v (fmt-locale self)))
-   "withLocale" (fn [self l] (formatter (fmt-pattern self) (locale-id l)))
+   "withLocale" (fn [self l] (impl/value :jolt.time/dt-formatter
+                                         {:pattern (impl/field self :pattern)
+                                          :style (impl/field self :style)
+                                          :locale (locale-id l)}))
    "withZone" (fn [self _z] self)
    "parse" (fn [self s] (let [f (parse-with-pattern (fmt-pattern self) (str s))]
                           (l/local-dt (days-from-civil (:year f) (:month f) (:day f))
@@ -149,23 +163,72 @@
    "SIMPLIFIED_CHINESE" (impl/value :jolt.time/locale {:id "zh-CN"}) "TRADITIONAL_CHINESE" (impl/value :jolt.time/locale {:id "zh-TW"})
    "UK" (impl/value :jolt.time/locale {:id "en-GB"}) "CANADA" (impl/value :jolt.time/locale {:id "en-CA"})
    "CANADA_FRENCH" (impl/value :jolt.time/locale {:id "fr-CA"}) "ROOT" (impl/value :jolt.time/locale {:id ""})})
-(__register-class-ctor! "Locale" (fn [lang & _] (impl/value :jolt.time/locale {:id (str lang)})))
-(__register-class-ctor! "java.util.Locale" (fn [lang & _] (impl/value :jolt.time/locale {:id (str lang)})))
+;; the two-arg ctor joins language and country like the constants do ("de" "DE"
+;; -> "de-DE"); the one-arg ctor's argument is a *language* subtag, so "en_US"
+;; stays malformed and resolves to ROOT at lookup time
+(defn- locale-ctor [lang & r]
+  (let [lang (str/lower-case (str lang))]
+    (if (and (seq r) (seq (str (first r))))
+      (impl/value :jolt.time/locale {:id (str lang "-" (str/upper-case (str (first r))))})
+      (impl/value :jolt.time/locale {:id lang}))))
+(__register-class-ctor! "Locale" locale-ctor)
+(__register-class-ctor! "java.util.Locale" locale-ctor)
 (impl/register-type! :jolt.time/locale
   {:eq (fn [a b] (= (impl/field a :id) (impl/field b :id))) :hash (fn [l] (hash (impl/field l :id)))
    :str (fn [l] (impl/field l :id)) :cmp nil :classes #{"java.util.Locale" "Locale"}})
 
-(def ^:private style-patterns
-  {:date {"SHORT" "M/d/yy" "MEDIUM" "MMM d, yyyy" "LONG" "MMMM d, yyyy" "FULL" "EEEE, MMMM d, yyyy"}
-   :time {"SHORT" "h:mm a" "MEDIUM" "h:mm:ss a" "LONG" "h:mm:ss a" "FULL" "h:mm:ss a"}
-   :datetime {"SHORT" "M/d/yy, h:mm a" "MEDIUM" "MMM d, yyyy, h:mm:ss a" "LONG" "MMMM d, yyyy, h:mm:ss a" "FULL" "EEEE, MMMM d, yyyy, h:mm:ss a"}})
+;; jolt core's java.text.SimpleDateFormat renders MMM/MMMM/EEE/EEEE through a
+;; :date-names extension point and carries ROOT alone, because core has no locale
+;; data of its own. Hand it the bundled tables so an explicit locale renders in its
+;; own language there too, not only through this library's DateTimeFormatter.
+;;
+;; Guarded on the point existing, so a jolt older than it still loads this library
+;; and simply keeps ROOT names in SimpleDateFormat. The guard is deliberately
+;; narrow: it swallows the one "not declared" case and rethrows anything else, so a
+;; genuine error in a provider still surfaces.
+;; The same applies to the other two locale-sensitive surfaces core declares:
+;; :number-symbols (String/format's decimal and grouping separators) and
+;; :currency-data (NumberFormat/getCurrencyInstance). All three are fed from the
+;; bundled tables here, because core carries ROOT alone for each.
+;;
+;; A point core does not declare is skipped rather than failing the load, so this
+;; library keeps working against a jolt that has only some of them.
+(defn- register-point! [reg point entries]
+  (try
+    (doseq [[id data] entries :when (not= id "")]
+      (reg point id data))
+    true
+    (catch Exception e
+      (if (re-find #"no extension point" (or (ex-message e) ""))
+        false
+        (throw e)))))
+
+(defn- register-locale-points! []
+  (when-let [reg (resolve 'jolt.host/register-extension!)]
+    (register-point! reg :date-names
+      (for [[id spec] ld/locales]
+        [id {:months (:months spec) :months-short (:months-short spec)
+             :days (:days spec) :days-short (:days-short spec)}]))
+    (register-point! reg :number-symbols nd/number-symbols)
+    (register-point! reg :currency-data nd/currency)
+    true))
+
+;; register-point! already tolerates an undeclared point per point, so this only
+;; has to run once.
+(defonce ^:private locale-points-registered (boolean (register-locale-points!)))
+
+(def ^:private style-key {"SHORT" :short "MEDIUM" :medium "LONG" :long "FULL" :full})
 (statics! ["FormatStyle" "java.time.format.FormatStyle"]
   (into {} (map (fn [s] [s (impl/value :jolt.time/format-style {:style s})]) ["SHORT" "MEDIUM" "LONG" "FULL"])))
 (impl/register-type! :jolt.time/format-style
   {:eq (fn [a b] (= (impl/field a :style) (impl/field b :style))) :hash (fn [s] (hash (impl/field s :style)))
    :str (fn [s] (impl/field s :style)) :cmp nil :classes #{"java.time.format.FormatStyle" "FormatStyle"}})
 (defn- style-of [fs] (impl/field fs :style))
-(defn- style-fmt [kind fs] (formatter (get-in style-patterns [kind (style-of fs)] "yyyy-MM-dd HH:mm:ss") "en"))
+;; ofLocalized* defers pattern resolution: the pattern depends on the locale,
+;; which .withLocale supplies after construction, so the value carries
+;; [kind style] and fmt-pattern resolves it per locale at use time.
+(defn- style-fmt [kind fs]
+  (impl/value :jolt.time/dt-formatter {:style [kind (style-key (style-of fs))] :locale "en"}))
 
 (statics! ["DateTimeFormatter" "java.time.format.DateTimeFormatter"]
   {"ofPattern" (fn [p & r] (formatter (str p) (if (seq r) (locale-id (first r)) "en")))
@@ -184,7 +247,7 @@
    "RFC_1123_DATE_TIME" (formatter "EEE, dd MMM yyyy HH:mm:ss Z" "en")
    "ofLocalizedDate" (fn [fs] (style-fmt :date fs))
    "ofLocalizedTime" (fn [fs] (style-fmt :time fs))
-   "ofLocalizedDateTime" (fn [fs] (style-fmt :datetime fs))})
+   "ofLocalizedDateTime" (fn [fs] (style-fmt :date-time fs))})
 
 ;; --- DateTimeFormatterBuilder (minimal) --------------------------------------
 (defn- builder [] (impl/value :jolt.time/dtf-builder {:pattern (atom "")}))
diff --git a/src/jolt/time/locale_data.clj b/src/jolt/time/locale_data.clj
new file mode 100644
index 0000000..bfdc883
--- /dev/null
+++ b/src/jolt/time/locale_data.clj
@@ -0,0 +1,627 @@
+(ns jolt.time.locale-data
+  "Localized date/time format data measured from the reference JVM's CLDR, plus
+  the lookups over it. Per locale id: :patterns keyed by [kind style] with kind
+  :date/:time/:date-time and style :short/:medium/:long/:full, and :months,
+  :months-short, :days, :days-short name tables (months January-first, days
+  Monday-first). Ids are BCP-47-ish \"lang-COUNTRY\"; \"\" is ROOT. Every value is
+  as the reference JVM renders it, including ROOT's wide month/day names, which
+  CLDR abbreviates ("Mar", "Sat") rather than spelling out.")
+
+(def locales
+  {""
+ {:patterns
+  {[:date-time :full] "y MMMM d, EEEE HH:mm:ss zzzz",
+   [:date-time :long] "y MMMM d HH:mm:ss z",
+   [:date :full] "y MMMM d, EEEE",
+   [:time :medium] "HH:mm:ss",
+   [:date :short] "y-MM-dd",
+   [:time :full] "HH:mm:ss zzzz",
+   [:time :long] "HH:mm:ss z",
+   [:date :medium] "y MMM d",
+   [:date :long] "y MMMM d",
+   [:date-time :short] "y-MM-dd HH:mm",
+   [:time :short] "HH:mm",
+   [:date-time :medium] "y MMM d HH:mm:ss"},
+  :months
+  ["Jan"
+   "Feb"
+   "Mar"
+   "Apr"
+   "May"
+   "Jun"
+   "Jul"
+   "Aug"
+   "Sep"
+   "Oct"
+   "Nov"
+   "Dec"],
+  :months-short
+  ["Jan"
+   "Feb"
+   "Mar"
+   "Apr"
+   "May"
+   "Jun"
+   "Jul"
+   "Aug"
+   "Sep"
+   "Oct"
+   "Nov"
+   "Dec"],
+  :days ["Mon" "Tue" "Wed" "Thu" "Fri" "Sat" "Sun"],
+  :days-short ["Mon" "Tue" "Wed" "Thu" "Fri" "Sat" "Sun"]},
+ "nl"
+ {:patterns
+  {[:date-time :full] "EEEE d MMMM y, HH:mm:ss zzzz",
+   [:date-time :long] "d MMMM y, HH:mm:ss z",
+   [:date :full] "EEEE d MMMM y",
+   [:time :medium] "HH:mm:ss",
+   [:date :short] "dd-MM-y",
+   [:time :full] "HH:mm:ss zzzz",
+   [:time :long] "HH:mm:ss z",
+   [:date :medium] "d MMM y",
+   [:date :long] "d MMMM y",
+   [:date-time :short] "dd-MM-y, HH:mm",
+   [:time :short] "HH:mm",
+   [:date-time :medium] "d MMM y, HH:mm:ss"},
+  :months
+  ["januari"
+   "februari"
+   "maart"
+   "april"
+   "mei"
+   "juni"
+   "juli"
+   "augustus"
+   "september"
+   "oktober"
+   "november"
+   "december"],
+  :months-short
+  ["jan"
+   "feb"
+   "mrt"
+   "apr"
+   "mei"
+   "jun"
+   "jul"
+   "aug"
+   "sep"
+   "okt"
+   "nov"
+   "dec"],
+  :days
+  ["maandag"
+   "dinsdag"
+   "woensdag"
+   "donderdag"
+   "vrijdag"
+   "zaterdag"
+   "zondag"],
+  :days-short ["ma" "di" "wo" "do" "vr" "za" "zo"]},
+ "pt"
+ {:patterns
+  {[:date-time :full] "EEEE, d 'de' MMMM 'de' y HH:mm:ss zzzz",
+   [:date-time :long] "d 'de' MMMM 'de' y HH:mm:ss z",
+   [:date :full] "EEEE, d 'de' MMMM 'de' y",
+   [:time :medium] "HH:mm:ss",
+   [:date :short] "dd/MM/y",
+   [:time :full] "HH:mm:ss zzzz",
+   [:time :long] "HH:mm:ss z",
+   [:date :medium] "d 'de' MMM 'de' y",
+   [:date :long] "d 'de' MMMM 'de' y",
+   [:date-time :short] "dd/MM/y HH:mm",
+   [:time :short] "HH:mm",
+   [:date-time :medium] "d 'de' MMM 'de' y HH:mm:ss"},
+  :months
+  ["janeiro"
+   "fevereiro"
+   "março"
+   "abril"
+   "maio"
+   "junho"
+   "julho"
+   "agosto"
+   "setembro"
+   "outubro"
+   "novembro"
+   "dezembro"],
+  :months-short
+  ["jan."
+   "fev."
+   "mar."
+   "abr."
+   "mai."
+   "jun."
+   "jul."
+   "ago."
+   "set."
+   "out."
+   "nov."
+   "dez."],
+  :days
+  ["segunda-feira"
+   "terça-feira"
+   "quarta-feira"
+   "quinta-feira"
+   "sexta-feira"
+   "sábado"
+   "domingo"],
+  :days-short ["seg." "ter." "qua." "qui." "sex." "sáb." "dom."]},
+ "en"
+ {:patterns
+  {[:date-time :full] "EEEE, MMMM d, y, h:mm:ss a zzzz",
+   [:date-time :long] "MMMM d, y, h:mm:ss a z",
+   [:date :full] "EEEE, MMMM d, y",
+   [:time :medium] "h:mm:ss a",
+   [:date :short] "M/d/yy",
+   [:time :full] "h:mm:ss a zzzz",
+   [:time :long] "h:mm:ss a z",
+   [:date :medium] "MMM d, y",
+   [:date :long] "MMMM d, y",
+   [:date-time :short] "M/d/yy, h:mm a",
+   [:time :short] "h:mm a",
+   [:date-time :medium] "MMM d, y, h:mm:ss a"},
+  :months
+  ["January"
+   "February"
+   "March"
+   "April"
+   "May"
+   "June"
+   "July"
+   "August"
+   "September"
+   "October"
+   "November"
+   "December"],
+  :months-short
+  ["Jan"
+   "Feb"
+   "Mar"
+   "Apr"
+   "May"
+   "Jun"
+   "Jul"
+   "Aug"
+   "Sep"
+   "Oct"
+   "Nov"
+   "Dec"],
+  :days
+  ["Monday"
+   "Tuesday"
+   "Wednesday"
+   "Thursday"
+   "Friday"
+   "Saturday"
+   "Sunday"],
+  :days-short ["Mon" "Tue" "Wed" "Thu" "Fri" "Sat" "Sun"]},
+ "zh"
+ {:patterns
+  {[:date-time :full] "y年M月d日EEEE zzzz HH:mm:ss",
+   [:date-time :long] "y年M月d日 z HH:mm:ss",
+   [:date :full] "y年M月d日EEEE",
+   [:time :medium] "HH:mm:ss",
+   [:date :short] "y/M/d",
+   [:time :full] "zzzz HH:mm:ss",
+   [:time :long] "z HH:mm:ss",
+   [:date :medium] "y年M月d日",
+   [:date :long] "y年M月d日",
+   [:date-time :short] "y/M/d HH:mm",
+   [:time :short] "HH:mm",
+   [:date-time :medium] "y年M月d日 HH:mm:ss"},
+  :months
+  ["一月" "二月" "三月" "四月" "五月" "六月" "七月" "八月" "九月" "十月" "十一月" "十二月"],
+  :months-short
+  ["1月" "2月" "3月" "4月" "5月" "6月" "7月" "8月" "9月" "10月" "11月" "12月"],
+  :days ["星期一" "星期二" "星期三" "星期四" "星期五" "星期六" "星期日"],
+  :days-short ["周一" "周二" "周三" "周四" "周五" "周六" "周日"]},
+ "it"
+ {:patterns
+  {[:date-time :full] "EEEE d MMMM y HH:mm:ss zzzz",
+   [:date-time :long] "d MMMM y HH:mm:ss z",
+   [:date :full] "EEEE d MMMM y",
+   [:time :medium] "HH:mm:ss",
+   [:date :short] "dd/MM/yy",
+   [:time :full] "HH:mm:ss zzzz",
+   [:time :long] "HH:mm:ss z",
+   [:date :medium] "d MMM y",
+   [:date :long] "d MMMM y",
+   [:date-time :short] "dd/MM/yy, HH:mm",
+   [:time :short] "HH:mm",
+   [:date-time :medium] "d MMM y, HH:mm:ss"},
+  :months
+  ["gennaio"
+   "febbraio"
+   "marzo"
+   "aprile"
+   "maggio"
+   "giugno"
+   "luglio"
+   "agosto"
+   "settembre"
+   "ottobre"
+   "novembre"
+   "dicembre"],
+  :months-short
+  ["gen"
+   "feb"
+   "mar"
+   "apr"
+   "mag"
+   "giu"
+   "lug"
+   "ago"
+   "set"
+   "ott"
+   "nov"
+   "dic"],
+  :days
+  ["lunedì"
+   "martedì"
+   "mercoledì"
+   "giovedì"
+   "venerdì"
+   "sabato"
+   "domenica"],
+  :days-short ["lun" "mar" "mer" "gio" "ven" "sab" "dom"]},
+ "fr"
+ {:patterns
+  {[:date-time :full] "EEEE d MMMM y, HH:mm:ss zzzz",
+   [:date-time :long] "d MMMM y, HH:mm:ss z",
+   [:date :full] "EEEE d MMMM y",
+   [:time :medium] "HH:mm:ss",
+   [:date :short] "dd/MM/y",
+   [:time :full] "HH:mm:ss zzzz",
+   [:time :long] "HH:mm:ss z",
+   [:date :medium] "d MMM y",
+   [:date :long] "d MMMM y",
+   [:date-time :short] "dd/MM/y HH:mm",
+   [:time :short] "HH:mm",
+   [:date-time :medium] "d MMM y, HH:mm:ss"},
+  :months
+  ["janvier"
+   "février"
+   "mars"
+   "avril"
+   "mai"
+   "juin"
+   "juillet"
+   "août"
+   "septembre"
+   "octobre"
+   "novembre"
+   "décembre"],
+  :months-short
+  ["janv."
+   "févr."
+   "mars"
+   "avr."
+   "mai"
+   "juin"
+   "juil."
+   "août"
+   "sept."
+   "oct."
+   "nov."
+   "déc."],
+  :days
+  ["lundi" "mardi" "mercredi" "jeudi" "vendredi" "samedi" "dimanche"],
+  :days-short ["lun." "mar." "mer." "jeu." "ven." "sam." "dim."]},
+ "de"
+ {:patterns
+  {[:date-time :full] "EEEE, d. MMMM y, HH:mm:ss zzzz",
+   [:date-time :long] "d. MMMM y, HH:mm:ss z",
+   [:date :full] "EEEE, d. MMMM y",
+   [:time :medium] "HH:mm:ss",
+   [:date :short] "dd.MM.yy",
+   [:time :full] "HH:mm:ss zzzz",
+   [:time :long] "HH:mm:ss z",
+   [:date :medium] "dd.MM.y",
+   [:date :long] "d. MMMM y",
+   [:date-time :short] "dd.MM.yy, HH:mm",
+   [:time :short] "HH:mm",
+   [:date-time :medium] "dd.MM.y, HH:mm:ss"},
+  :months
+  ["Januar"
+   "Februar"
+   "März"
+   "April"
+   "Mai"
+   "Juni"
+   "Juli"
+   "August"
+   "September"
+   "Oktober"
+   "November"
+   "Dezember"],
+  :months-short
+  ["Jan."
+   "Feb."
+   "März"
+   "Apr."
+   "Mai"
+   "Juni"
+   "Juli"
+   "Aug."
+   "Sept."
+   "Okt."
+   "Nov."
+   "Dez."],
+  :days
+  ["Montag"
+   "Dienstag"
+   "Mittwoch"
+   "Donnerstag"
+   "Freitag"
+   "Samstag"
+   "Sonntag"],
+  :days-short ["Mo." "Di." "Mi." "Do." "Fr." "Sa." "So."]},
+ "ru"
+ {:patterns
+  {[:date-time :full] "EEEE, d MMMM y 'г'., HH:mm:ss zzzz",
+   [:date-time :long] "d MMMM y 'г'., HH:mm:ss z",
+   [:date :full] "EEEE, d MMMM y 'г'.",
+   [:time :medium] "HH:mm:ss",
+   [:date :short] "dd.MM.y",
+   [:time :full] "HH:mm:ss zzzz",
+   [:time :long] "HH:mm:ss z",
+   [:date :medium] "d MMM y 'г'.",
+   [:date :long] "d MMMM y 'г'.",
+   [:date-time :short] "dd.MM.y, HH:mm",
+   [:time :short] "HH:mm",
+   [:date-time :medium] "d MMM y 'г'., HH:mm:ss"},
+  :months
+  ["января"
+   "февраля"
+   "марта"
+   "апреля"
+   "мая"
+   "июня"
+   "июля"
+   "августа"
+   "сентября"
+   "октября"
+   "ноября"
+   "декабря"],
+  :months-short
+  ["янв."
+   "февр."
+   "мар."
+   "апр."
+   "мая"
+   "июн."
+   "июл."
+   "авг."
+   "сент."
+   "окт."
+   "нояб."
+   "дек."],
+  :days
+  ["понедельник"
+   "вторник"
+   "среда"
+   "четверг"
+   "пятница"
+   "суббота"
+   "воскресенье"],
+  :days-short ["пн" "вт" "ср" "чт" "пт" "сб" "вс"]},
+ "es"
+ {:patterns
+  {[:date-time :full] "EEEE, d 'de' MMMM 'de' y, H:mm:ss (zzzz)",
+   [:date-time :long] "d 'de' MMMM 'de' y, H:mm:ss z",
+   [:date :full] "EEEE, d 'de' MMMM 'de' y",
+   [:time :medium] "H:mm:ss",
+   [:date :short] "d/M/yy",
+   [:time :full] "H:mm:ss (zzzz)",
+   [:time :long] "H:mm:ss z",
+   [:date :medium] "d MMM y",
+   [:date :long] "d 'de' MMMM 'de' y",
+   [:date-time :short] "d/M/yy, H:mm",
+   [:time :short] "H:mm",
+   [:date-time :medium] "d MMM y, H:mm:ss"},
+  :months
+  ["enero"
+   "febrero"
+   "marzo"
+   "abril"
+   "mayo"
+   "junio"
+   "julio"
+   "agosto"
+   "septiembre"
+   "octubre"
+   "noviembre"
+   "diciembre"],
+  :months-short
+  ["ene"
+   "feb"
+   "mar"
+   "abr"
+   "may"
+   "jun"
+   "jul"
+   "ago"
+   "sept"
+   "oct"
+   "nov"
+   "dic"],
+  :days
+  ["lunes" "martes" "miércoles" "jueves" "viernes" "sábado" "domingo"],
+  :days-short ["lun" "mar" "mié" "jue" "vie" "sáb" "dom"]},
+ "en-US"
+ {:patterns
+  {[:date-time :full] "EEEE, MMMM d, y, h:mm:ss a zzzz",
+   [:date-time :long] "MMMM d, y, h:mm:ss a z",
+   [:date :full] "EEEE, MMMM d, y",
+   [:time :medium] "h:mm:ss a",
+   [:date :short] "M/d/yy",
+   [:time :full] "h:mm:ss a zzzz",
+   [:time :long] "h:mm:ss a z",
+   [:date :medium] "MMM d, y",
+   [:date :long] "MMMM d, y",
+   [:date-time :short] "M/d/yy, h:mm a",
+   [:time :short] "h:mm a",
+   [:date-time :medium] "MMM d, y, h:mm:ss a"},
+  :months
+  ["January"
+   "February"
+   "March"
+   "April"
+   "May"
+   "June"
+   "July"
+   "August"
+   "September"
+   "October"
+   "November"
+   "December"],
+  :months-short
+  ["Jan"
+   "Feb"
+   "Mar"
+   "Apr"
+   "May"
+   "Jun"
+   "Jul"
+   "Aug"
+   "Sep"
+   "Oct"
+   "Nov"
+   "Dec"],
+  :days
+  ["Monday"
+   "Tuesday"
+   "Wednesday"
+   "Thursday"
+   "Friday"
+   "Saturday"
+   "Sunday"],
+  :days-short ["Mon" "Tue" "Wed" "Thu" "Fri" "Sat" "Sun"]},
+ "ja"
+ {:patterns
+  {[:date-time :full] "y年M月d日EEEE H時mm分ss秒 zzzz",
+   [:date-time :long] "y年M月d日 H:mm:ss z",
+   [:date :full] "y年M月d日EEEE",
+   [:time :medium] "H:mm:ss",
+   [:date :short] "y/MM/dd",
+   [:time :full] "H時mm分ss秒 zzzz",
+   [:time :long] "H:mm:ss z",
+   [:date :medium] "y/MM/dd",
+   [:date :long] "y年M月d日",
+   [:date-time :short] "y/MM/dd H:mm",
+   [:time :short] "H:mm",
+   [:date-time :medium] "y/MM/dd H:mm:ss"},
+  :months
+  ["1月" "2月" "3月" "4月" "5月" "6月" "7月" "8月" "9月" "10月" "11月" "12月"],
+  :months-short
+  ["1月" "2月" "3月" "4月" "5月" "6月" "7月" "8月" "9月" "10月" "11月" "12月"],
+  :days ["月曜日" "火曜日" "水曜日" "木曜日" "金曜日" "土曜日" "日曜日"],
+  :days-short ["月" "火" "水" "木" "金" "土" "日"]},
+ "en-GB"
+ {:patterns
+  {[:date-time :full] "EEEE, d MMMM y, HH:mm:ss zzzz",
+   [:date-time :long] "d MMMM y, HH:mm:ss z",
+   [:date :full] "EEEE, d MMMM y",
+   [:time :medium] "HH:mm:ss",
+   [:date :short] "dd/MM/y",
+   [:time :full] "HH:mm:ss zzzz",
+   [:time :long] "HH:mm:ss z",
+   [:date :medium] "d MMM y",
+   [:date :long] "d MMMM y",
+   [:date-time :short] "dd/MM/y, HH:mm",
+   [:time :short] "HH:mm",
+   [:date-time :medium] "d MMM y, HH:mm:ss"},
+  :months
+  ["January"
+   "February"
+   "March"
+   "April"
+   "May"
+   "June"
+   "July"
+   "August"
+   "September"
+   "October"
+   "November"
+   "December"],
+  :months-short
+  ["Jan"
+   "Feb"
+   "Mar"
+   "Apr"
+   "May"
+   "Jun"
+   "Jul"
+   "Aug"
+   "Sept"
+   "Oct"
+   "Nov"
+   "Dec"],
+  :days
+  ["Monday"
+   "Tuesday"
+   "Wednesday"
+   "Thursday"
+   "Friday"
+   "Saturday"
+   "Sunday"],
+  :days-short ["Mon" "Tue" "Wed" "Thu" "Fri" "Sat" "Sun"]},
+ "ko"
+ {:patterns
+  {[:date-time :full] "y년 MMMM d일 EEEE a h시 m분 s초 zzzz",
+   [:date-time :long] "y년 MMMM d일 a h시 m분 s초 z",
+   [:date :full] "y년 MMMM d일 EEEE",
+   [:time :medium] "a h:mm:ss",
+   [:date :short] "yy. M. d.",
+   [:time :full] "a h시 m분 s초 zzzz",
+   [:time :long] "a h시 m분 s초 z",
+   [:date :medium] "y. M. d.",
+   [:date :long] "y년 MMMM d일",
+   [:date-time :short] "yy. M. d. a h:mm",
+   [:time :short] "a h:mm",
+   [:date-time :medium] "y. M. d. a h:mm:ss"},
+  :months
+  ["1월" "2월" "3월" "4월" "5월" "6월" "7월" "8월" "9월" "10월" "11월" "12월"],
+  :months-short
+  ["1월" "2월" "3월" "4월" "5월" "6월" "7월" "8월" "9월" "10월" "11월" "12월"],
+  :days ["월요일" "화요일" "수요일" "목요일" "금요일" "토요일" "일요일"],
+  :days-short ["월" "화" "수" "목" "금" "토" "일"]}}
+)
+
+(defn- last-hyphen [s]
+  (loop [i (dec (count s))]
+    (cond (neg? i) nil
+          (= \- (nth s i)) i
+          :else (recur (dec i)))))
+
+(defn- has-underscore? [s]
+  (loop [i 0]
+    (cond (>= i (count s)) false
+          (= \_ (nth s i)) true
+          :else (recur (inc i)))))
+
+(defn resolve-id
+  "Narrow a locale id to the most specific id the tables carry: \"zh-CN\" ->
+  \"zh\" -> \"\". An id containing an underscore is a malformed language subtag
+  (e.g. (Locale. \"en_US\")) and lands on ROOT, matching the JVM."
+  [id]
+  (let [id (or id "")]
+    (cond
+      (contains? locales id) id
+      (has-underscore? id) ""
+      :else (loop [s id]
+              (if-let [i (last-hyphen s)]
+                (let [p (subs s 0 i)]
+                  (if (contains? locales p) p (recur p)))
+                "")))))
+
+(defn pattern
+  "Localized pattern for [kind style]; kind :date/:time/:date-time, style
+  :short/:medium/:long/:full. Always non-nil: ROOT carries all twelve."
+  [id kind style]
+  (get-in locales [(resolve-id id) :patterns [kind style]]))
+
+(defn months [id full?] (get-in locales [(resolve-id id) (if full? :months :months-short)]))
+(defn days [id full?] (get-in locales [(resolve-id id) (if full? :days :days-short)]))
diff --git a/src/jolt/time/number_data.clj b/src/jolt/time/number_data.clj
new file mode 100644
index 0000000..2ef2521
--- /dev/null
+++ b/src/jolt/time/number_data.clj
@@ -0,0 +1,76 @@
+(ns jolt.time.number-data
+  "Per-locale number and currency symbols, measured from the reference JVM.
+
+  Feeds jolt core's :number-symbols and :currency-data extension points, which
+  core declares with ROOT values only because it carries no locale data of its
+  own. Ids are BCP-47-ish lang-COUNTRY; the empty id is ROOT.
+
+  Note a language-only id has no country and therefore no currency: the JVM
+  renders de as the generic currency sign, and only a country id such as
+  de-DE carries a real symbol. Both are here, as measured -- every currency
+  entry was checked by rebuilding the JVM's formatted output from these fields."
+  (:require [jolt.time.locale-data :as ld]))
+
+(def number-symbols
+  {""       {:decimal-sep "." :grouping-sep ","}
+   "de"     {:decimal-sep "," :grouping-sep "."}
+   "de-CH"  {:decimal-sep "." :grouping-sep "'"}
+   "de-DE"  {:decimal-sep "," :grouping-sep "."}
+   "en"     {:decimal-sep "." :grouping-sep ","}
+   "en-GB"  {:decimal-sep "." :grouping-sep ","}
+   "en-US"  {:decimal-sep "." :grouping-sep ","}
+   "es"     {:decimal-sep "," :grouping-sep "."}
+   "es-ES"  {:decimal-sep "," :grouping-sep "."}
+   "fr"     {:decimal-sep "," :grouping-sep " "}
+   "fr-FR"  {:decimal-sep "," :grouping-sep " "}
+   "it"     {:decimal-sep "," :grouping-sep "."}
+   "it-IT"  {:decimal-sep "," :grouping-sep "."}
+   "ja"     {:decimal-sep "." :grouping-sep ","}
+   "ja-JP"  {:decimal-sep "." :grouping-sep ","}
+   "ko"     {:decimal-sep "." :grouping-sep ","}
+   "ko-KR"  {:decimal-sep "." :grouping-sep ","}
+   "nl"     {:decimal-sep "," :grouping-sep "."}
+   "nl-NL"  {:decimal-sep "," :grouping-sep "."}
+   "pt"     {:decimal-sep "," :grouping-sep "."}
+   "pt-BR"  {:decimal-sep "," :grouping-sep "."}
+   "ru"     {:decimal-sep "," :grouping-sep " "}
+   "ru-RU"  {:decimal-sep "," :grouping-sep " "}
+   "zh"     {:decimal-sep "." :grouping-sep ","}
+   "zh-CN"  {:decimal-sep "." :grouping-sep ","}})
+
+(def currency
+  {""       {:symbol "¤", :symbol-sep " ", :symbol-first? true, :frac-digits 2, :decimal-sep ".", :grouping-sep ","}
+   "de"     {:symbol "¤", :symbol-sep " ", :symbol-first? false, :frac-digits 2, :decimal-sep ",", :grouping-sep "."}
+   "de-CH"  {:symbol "CHF", :symbol-sep " ", :symbol-first? true, :frac-digits 2, :decimal-sep ".", :grouping-sep "'"}
+   "de-DE"  {:symbol "€", :symbol-sep " ", :symbol-first? false, :frac-digits 2, :decimal-sep ",", :grouping-sep "."}
+   "en"     {:symbol "¤", :symbol-sep "", :symbol-first? true, :frac-digits 2, :decimal-sep ".", :grouping-sep ","}
+   "en-GB"  {:symbol "£", :symbol-sep "", :symbol-first? true, :frac-digits 2, :decimal-sep ".", :grouping-sep ","}
+   "en-US"  {:symbol "$", :symbol-sep "", :symbol-first? true, :frac-digits 2, :decimal-sep ".", :grouping-sep ","}
+   "es"     {:symbol "¤", :symbol-sep " ", :symbol-first? false, :frac-digits 2, :decimal-sep ",", :grouping-sep "."}
+   "es-ES"  {:symbol "€", :symbol-sep " ", :symbol-first? false, :frac-digits 2, :decimal-sep ",", :grouping-sep "."}
+   "fr"     {:symbol "¤", :symbol-sep " ", :symbol-first? false, :frac-digits 2, :decimal-sep ",", :grouping-sep " "}
+   "fr-FR"  {:symbol "€", :symbol-sep " ", :symbol-first? false, :frac-digits 2, :decimal-sep ",", :grouping-sep " "}
+   "it"     {:symbol "¤", :symbol-sep " ", :symbol-first? false, :frac-digits 2, :decimal-sep ",", :grouping-sep "."}
+   "it-IT"  {:symbol "€", :symbol-sep " ", :symbol-first? false, :frac-digits 2, :decimal-sep ",", :grouping-sep "."}
+   "ja"     {:symbol "¤", :symbol-sep "", :symbol-first? true, :frac-digits 2, :decimal-sep ".", :grouping-sep ","}
+   "ja-JP"  {:symbol "￥", :symbol-sep "", :symbol-first? true, :frac-digits 0, :decimal-sep ".", :grouping-sep ","}
+   "ko"     {:symbol "¤", :symbol-sep "", :symbol-first? true, :frac-digits 2, :decimal-sep ".", :grouping-sep ","}
+   "ko-KR"  {:symbol "₩", :symbol-sep "", :symbol-first? true, :frac-digits 0, :decimal-sep ".", :grouping-sep ","}
+   "nl"     {:symbol "¤", :symbol-sep " ", :symbol-first? true, :frac-digits 2, :decimal-sep ",", :grouping-sep "."}
+   "nl-NL"  {:symbol "€", :symbol-sep " ", :symbol-first? true, :frac-digits 2, :decimal-sep ",", :grouping-sep "."}
+   "pt"     {:symbol "¤", :symbol-sep " ", :symbol-first? true, :frac-digits 2, :decimal-sep ",", :grouping-sep "."}
+   "pt-BR"  {:symbol "R$", :symbol-sep " ", :symbol-first? true, :frac-digits 2, :decimal-sep ",", :grouping-sep "."}
+   "ru"     {:symbol "¤", :symbol-sep " ", :symbol-first? false, :frac-digits 2, :decimal-sep ",", :grouping-sep " "}
+   "ru-RU"  {:symbol "₽", :symbol-sep " ", :symbol-first? false, :frac-digits 2, :decimal-sep ",", :grouping-sep " "}
+   "zh"     {:symbol "¤", :symbol-sep "", :symbol-first? true, :frac-digits 2, :decimal-sep ".", :grouping-sep ","}
+   "zh-CN"  {:symbol "¥", :symbol-sep "", :symbol-first? true, :frac-digits 2, :decimal-sep ".", :grouping-sep ","}})
+
+(defn symbols-for
+  "Number symbols for a locale id, narrowed the same way the pattern tables are."
+  [id]
+  (get number-symbols (ld/resolve-id id) (get number-symbols "")))
+
+(defn currency-for
+  "Currency data for a locale id, narrowed the same way."
+  [id]
+  (get currency (ld/resolve-id id) (get currency "")))
diff --git a/test/jolt/time/fmt_test.clj b/test/jolt/time/fmt_test.clj
index 2c66301..edcada5 100644
--- a/test/jolt/time/fmt_test.clj
+++ b/test/jolt/time/fmt_test.clj
@@ -25,5 +25,43 @@
   (is (= "Mar 5, 2020" (.format (DateTimeFormatter/ofLocalizedDate FormatStyle/MEDIUM) (LocalDate/of 2020 3 5))))
   (is (string? (.format (.withLocale (DateTimeFormatter/ofPattern "yyyy") (java.util.Locale. "en")) (LocalDate/of 2020 1 1)))))
 
+(deftest localized-per-locale
+  ;; Selmer's locale date assertions, as produced by the reference JVM; the date
+  ;; is 2014-03-01T00:00. Note (java.util.Locale. "en_US") is a malformed
+  ;; *language* subtag and resolves to ROOT (ISO-ish patterns), not to English
+  ;; and not to US.
+  (let [d (LocalDateTime/of 2014 3 1 0 0 0)
+        fmt (fn [f id] (.format (.withLocale f (java.util.Locale. id)) d))]
+    ;; month name from the bundled table; no OS locale needed
+    (is (= "mars" (fmt (DateTimeFormatter/ofPattern "MMMM") "fr")))
+    (is (= "00:00" (fmt (DateTimeFormatter/ofLocalizedTime FormatStyle/SHORT) "en_US")))
+    (is (= "00:00" (fmt (DateTimeFormatter/ofLocalizedTime FormatStyle/SHORT) "zh")))
+    (is (= "2014-03-01" (fmt (DateTimeFormatter/ofLocalizedDate FormatStyle/SHORT) "en_US")))
+    (is (= "2014/3/1" (fmt (DateTimeFormatter/ofLocalizedDate FormatStyle/SHORT) "zh")))
+    (is (= "2014-03-01 00:00" (fmt (DateTimeFormatter/ofLocalizedDateTime FormatStyle/SHORT) "en_US")))
+    (is (= "2014/3/1 00:00" (fmt (DateTimeFormatter/ofLocalizedDateTime FormatStyle/SHORT) "zh")))
+    (is (= "2014年3月1日 00:00:00" (fmt (DateTimeFormatter/ofLocalizedDateTime FormatStyle/MEDIUM) "zh")))
+    (is (= "2014年3月1日" (fmt (DateTimeFormatter/ofLocalizedDate FormatStyle/LONG) "zh")))
+    ;; ROOT wide month is "Mar", not "March" — CLDR ROOT abbreviates, verified on the
+    ;; reference JVM. Selmer accepts either shape here and says so in its own comment.
+    (is (= "2014 Mar 1" (fmt (DateTimeFormatter/ofLocalizedDate FormatStyle/LONG) "en_US")))
+    ;; day name from the bundled table; 2014-03-01 is a Saturday
+    (is (= "星期六" (fmt (DateTimeFormatter/ofPattern "EEEE") "zh")))))
+
+(deftest localized-resolution
+  (let [d (LocalDate/of 2014 3 1)
+        short-date (fn [locale] (.format (.withLocale (DateTimeFormatter/ofLocalizedDate FormatStyle/SHORT) locale) d))]
+    ;; narrowing: zh-CN has no entry of its own and falls back to zh
+    (is (= "2014年3月1日"
+           (.format (.withLocale (DateTimeFormatter/ofLocalizedDate FormatStyle/LONG) (java.util.Locale. "zh" "CN")) d)))
+    ;; a well-formed but unknown id lands on ROOT
+    (is (= "2014-03-01" (short-date (java.util.Locale. "tlh"))))
+    (is (= "2014-03-01" (short-date (java.util.Locale. "xx" "YY"))))
+    ;; US proper is not ROOT
+    (is (= "3/1/14" (short-date (java.util.Locale. "en" "US"))))
+    ;; the two-arg ctor joins language and country like the constants do
+    (is (= "01.03.14" (short-date (java.util.Locale. "de" "DE"))))
+    (is (= (short-date (java.util.Locale. "de" "DE")) (short-date java.util.Locale/GERMANY)))))
+
 (deftest via-temporal-method
   (is (= "2020-03-05" (.format (LocalDate/of 2020 3 5) (DateTimeFormatter/ofPattern "yyyy-MM-dd")))))