diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 1932b8e..5f4dbe6 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -4,7 +4,6 @@ name: ci
 # step when either changes.
 on:
   pull_request:
-    branches: [main]
   push:
     branches: [main]
 
diff --git a/.github/workflows/commitlint.yml b/.github/workflows/commitlint.yml
index 7b0e22a..6d922d6 100644
--- a/.github/workflows/commitlint.yml
+++ b/.github/workflows/commitlint.yml
@@ -3,7 +3,6 @@ name: lint-commits
 # Enforces conventional commit titles so git-cliff can drive the release notes. See cliff.toml for the type groups.
 on:
   pull_request:
-    branches: [main]
   push:
     branches: [main]
 
diff --git a/docs/data-format.md b/docs/data-format.md
index 5d98c7e..b24b525 100644
--- a/docs/data-format.md
+++ b/docs/data-format.md
@@ -119,6 +119,10 @@ Rules:
 
 ### Objects
 
+Several products may retain different file roles of the same object. Loading their manifests combines these
+roles; overlapping roles and object metadata must agree. Product paths may include region directories, and are
+relative to the layout root.
+
 An object is one Unity object, converted. Its id is `<serialized file>:<path id>` (the path id as decimal digits,
 possibly negative), the same in every product that uses it. An object's JSON names another object by a reference
 `{ "$ref": { "file": "<serialized file>", "pathId": "<path id>" } }`, the object `<file>:<pathId>`; `null` names none.
@@ -180,6 +184,9 @@ A chart manifest of a [chart site](#chart-sites) is read as a live as well.
 | `notesMirror` | Optional. The score converted mirrored (the game's `MirrorChart` option: lanes and flick directions mirrored while the score is converted), same format as `notes`. Without it the `MirrorChart` option is not offered. |
 | `parts` | Part → the manifest path of the product, relative to the root ([Lives](#lives)). |
 
+The optional `live.json` field `parts.ui` maps language codes to UI product manifests. These entries are
+language-specific resources; the shared scene, score and audio parts remain ordinary manifest paths.
+
 ## Score
 
 The file `live.json` `notes` names ([schema](../schema/score.schema.json)): the notes and lines of one chart, as the
diff --git a/src/data/products.js b/src/data/products.js
new file mode 100644
index 0000000..7a6c831
--- /dev/null
+++ b/src/data/products.js
@@ -0,0 +1,55 @@
+import { PRODUCT_MANIFEST } from "./assets.js";
+
+// The directory holding a product's files within an AssetStore.
+export const productDir = (manifestPath) => {
+  const p = String(manifestPath), parts = p.split("/");
+  if (parts.length < 2 || parts.some((s) => !s || s === "." || s === "..") || !p.endsWith(".json") || /[?#\\]/.test(p))
+    throw new Error(`${p}: not a product manifest path <directory>/<file>.json`);
+  return p.slice(0, -".json".length);
+};
+
+export const isProduct = (m, product) => !!m && m.schema === PRODUCT_MANIFEST && m.product === product
+  && !!m.files && typeof m.files === "object" && !Array.isArray(m.files)
+  && !!m.objects && typeof m.objects === "object" && !Array.isArray(m.objects);
+
+const same = (a, b) => {
+  if (a === b) return true;
+  if (!a || !b || typeof a !== "object" || typeof b !== "object" || Array.isArray(a) !== Array.isArray(b)) return false;
+  const keys = Object.keys(a);
+  return keys.length === Object.keys(b).length && keys.every((k) => Object.hasOwn(b, k) && same(a[k], b[k]));
+};
+
+// Merge compatible object roles. Each product may retain a different subset of one object's files.
+export const mergeProductManifests = (loaded) => {
+  if (!loaded.length) throw new Error("no product manifests");
+  const files = {}, objects = {}, products = {};
+  for (const [dir, m] of loaded) {
+    const subjects = (products[m.product] ||= {});
+    if (m.subject in subjects) {
+      if (subjects[m.subject] !== dir) throw new Error(`${m.product} ${m.subject}: two manifests (${subjects[m.subject]}, ${dir})`);
+    }
+    subjects[m.subject] = dir;
+    for (const [p, f] of Object.entries(m.files)) {
+      const path = `${dir}/${p}`;
+      if (Object.hasOwn(files, path) && !same(files[path], f)) throw new Error(`${path}: differing product files`);
+      files[path] = f;
+    }
+    for (const [id, o] of Object.entries(m.objects)) {
+      const target = (objects[id] ||= { files: {} });
+      for (const [key, value] of Object.entries(o)) {
+        if (key === "files") continue;
+        if (Object.hasOwn(target, key) && !same(target[key], value))
+          throw new Error(`object ${id} differs between the manifests of the products (${key})`);
+        target[key] = value;
+      }
+      for (const [role, file] of Object.entries(o.files || {})) {
+        if (Object.hasOwn(target.files, role) && !same(target.files[role], file))
+          throw new Error(`object ${id} differs between the manifests of the products (${role})`);
+        target.files[role] = file;
+      }
+    }
+  }
+  const [dir, man] = loaded[0];
+  const { files: _f, objects: _o, ...info } = man;
+  return { ...info, dir, products, files, objects };
+};
diff --git a/src/live/data.js b/src/live/data.js
index 36d94c3..14b9848 100644
--- a/src/live/data.js
+++ b/src/live/data.js
@@ -27,7 +27,8 @@ export class LiveData {
     this.dir = info.dir;
     this.products = info.products;
     this.live = checkFormat(assets.json(`${this.dir}/live.json`), LIVE_JSON_FORMAT, `${this.dir}/live.json`);
-    this.parts = Object.fromEntries(Object.entries(this.live.parts).map(([part, p]) => [part, productDir(p)]));
+    this.parts = Object.fromEntries(Object.entries(this.live.parts).filter(([, p]) => typeof p === "string")
+      .map(([part, p]) => [part, productDir(p)]));
     this._docs = new Map();
   }
 
diff --git a/src/live/store.js b/src/live/store.js
index 7f4f345..7d10efd 100644
--- a/src/live/store.js
+++ b/src/live/store.js
@@ -1,4 +1,6 @@
 import { AssetStore, PRODUCT_MANIFEST, decodeAsset } from "../data/assets.js";
+import { isProduct, mergeProductManifests, productDir } from "../data/products.js";
+export { productDir } from "../data/products.js";
 
 // Loading a live: the product manifest of the live (lives/<musicId>_<difficulty>.json, the `live` product of nnnotes'
 // product layout), the manifests of the products its live.json `parts` names, and any further products of note skins,
@@ -17,16 +19,6 @@ export const LIVE_VARIANTS = Object.freeze(["live.noteskin", "live.noteeffect",
 
 const here = () => (globalThis.document ? globalThis.document.baseURI : globalThis.location ? globalThis.location.href : undefined);
 
-// the directory of a product's files in a live store: its manifest path without ".json"
-export const productDir = (manifestPath) => {
-  const p = String(manifestPath);
-  if (!/^[^/]+\/[^/]+\.json$/.test(p)) throw new Error(`${p}: not a product manifest path <directory>/<file>.json`);
-  return p.slice(0, -".json".length);
-};
-
-const isProduct = (m, product) => !!m && m.schema === PRODUCT_MANIFEST && m.product === product
-  && !!m.files && typeof m.files === "object" && !!m.objects && typeof m.objects === "object";
-
 // A chart manifest of the site layout (docs/data-format.md "Chart manifest"): no `schema`, a `live.json` file. Its
 // store is read through legacy.js.
 export const isChartManifest = (m) => !!m && m.schema === undefined && !!m.files && !!m.files["live.json"];
@@ -86,23 +78,4 @@ export const loadLiveStore = async (url, { variants = [], fetch = globalThis.fet
 // first: the live manifest without its file lists, `dir` and `products` (as the store's `info` of loadLiveStore), every
 // product's files under its directory and the objects of all of them in one table (an object in several manifests
 // must be the same in each).
-export const liveManifest = (loaded) => {
-  const files = {}, objects = {}, products = {};
-  for (const [dir, m] of loaded) {
-    const subjects = (products[m.product] ||= {});
-    if (m.subject in subjects) {
-      if (subjects[m.subject] !== dir) throw new Error(`${m.product} ${m.subject}: two manifests (${subjects[m.subject]}, ${dir})`);
-      continue;                                            // a variant that is also a part
-    }
-    subjects[m.subject] = dir;
-    for (const [p, f] of Object.entries(m.files)) files[`${dir}/${p}`] = f;
-    for (const [id, o] of Object.entries(m.objects)) {
-      if (id in objects && JSON.stringify(objects[id]) !== JSON.stringify(o))
-        throw new Error(`object ${id} differs between the manifests of the live's products`);
-      objects[id] = o;
-    }
-  }
-  const [liveDir, man] = loaded[0];
-  const { files: _f, objects: _o, ...info } = man;
-  return { ...info, dir: liveDir, products, files, objects };
-};
+export const liveManifest = mergeProductManifests;
diff --git a/tests/unit/live-store.test.mjs b/tests/unit/live-store.test.mjs
index a5c1dbe..2239303 100644
--- a/tests/unit/live-store.test.mjs
+++ b/tests/unit/live-store.test.mjs
@@ -66,10 +66,26 @@ const live = () => {
   return L;
 };
 
-test("productDir: a manifest path without .json, one directory and a file", () => {
+test("productDir: a product manifest path without .json", () => {
   assert.equal(productDir("scenes/default.json"), "scenes/default");
   assert.equal(productDir("sounds/NoteSePatternId=1,3.json"), "sounds/NoteSePatternId=1,3");
-  for (const bad of ["scene.json", "a/b/c.json", "scenes/default"]) assert.throws(() => productDir(bad), /not a product manifest path/);
+  assert.equal(productDir("region/scenes/default.json"), "region/scenes/default");
+  for (const bad of ["scene.json", "a/../c.json", "scenes/default"])
+    assert.throws(() => productDir(bad), /not a product manifest path/);
+});
+
+test("LiveData accepts language-indexed UI parts beside shared parts", async () => {
+  const L = live();
+  const manifest = L.files.get("lives/7_expert.json"), path = manifest.files["live.json"].asset;
+  const doc = JSON.parse(new TextDecoder().decode(L.files.get(path)));
+  doc.parts.ui = { ja: "liveui/7_expert_ja.json", en: "liveui/7_expert_en.json" };
+  const bytes = new TextEncoder().encode(JSON.stringify(doc));
+  L.files.set(path, bytes);
+  manifest.files["live.json"].size = bytes.byteLength;
+  const data = new LiveData(await loadLiveStore(`${ROOT}lives/7_expert.json`, { fetch: L.fetch }));
+  assert.equal(data.scene.format, 1);
+  assert.deepEqual(data.score().notes, [1]);
+  assert.equal(data.live.parts.ui.ja, "liveui/7_expert_ja.json");
 });
 
 test("loadLiveStore: the live, its parts and the variants, each under its directory, one object table", async () => {
@@ -194,4 +210,4 @@ test("legacyLive: the note assets, the skin's arrow clip and the effect sets' re
     assert.deepEqual(ps.m_VertexStreams, { $hex: "00010304" });
   }
   assert.equal(store.mono(data.scene.spriteAssets.combo).$script, "LiveComboSpriteAsset");
-});
\ No newline at end of file
+});
diff --git a/tests/unit/product-manifests.test.mjs b/tests/unit/product-manifests.test.mjs
new file mode 100644
index 0000000..f32f4fc
--- /dev/null
+++ b/tests/unit/product-manifests.test.mjs
@@ -0,0 +1,43 @@
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import { mergeProductManifests, productDir } from "../../src/data/products.js";
+
+const json = { asset: "assets/texture.json", size: 2 };
+const image = { asset: "assets/texture.png", size: 17 };
+const product = (name, files) => [name, {
+  schema: "nnnotes.product/1", product: name, subject: "default", files: {},
+  objects: { "cab:1": { class: "Texture2D", name: "texture", files } },
+}];
+
+test("product objects combine complementary roles and compare file fields by value", () => {
+  const left = product("scene", { json });
+  const right = product("ui", { image, json: { size: 2, asset: json.asset } });
+  const result = mergeProductManifests([left, right]);
+  assert.deepEqual(result.objects["cab:1"].files, { json, image });
+  assert.deepEqual(left[1].objects["cab:1"].files, { json });
+  assert.deepEqual(result.products, { scene: { default: "scene" }, ui: { default: "ui" } });
+});
+
+test("shared object metadata and overlapping file roles must agree", () => {
+  const left = product("scene", { json });
+  const right = product("ui", { json: { ...json, size: 3 } });
+  assert.throws(() => mergeProductManifests([left, right]), /object cab:1 differs.*json/);
+  right[1].objects["cab:1"].files = { json };
+  right[1].objects["cab:1"].class = "Material";
+  assert.throws(() => mergeProductManifests([left, right]), /object cab:1 differs.*class/);
+});
+
+test("duplicate product paths retain the same file content", () => {
+  const left = product("scene", { json });
+  left[1].files = { "scene.json": json };
+  const right = structuredClone(left);
+  right[1].files["scene.json"].size++;
+  assert.throws(() => mergeProductManifests([left, right]), /differing product files/);
+  assert.throws(() => mergeProductManifests([]), /no product manifests/);
+});
+
+test("product paths may include region directories and must remain relative", () => {
+  assert.equal(productDir("regions/a/stories/7.json"), "regions/a/stories/7");
+  for (const path of ["/stories/7.json", "stories/../7.json", "stories/7.json?x", "a//7.json", "a\\7.json"])
+    assert.throws(() => productDir(path), /not a product manifest path/);
+});