Add Simplified Chinese localization (String Catalog) + in-app language switcher

[MJorgin]

What this does

Adds a Simplified Chinese (zh-Hans) localization to Compositor, and a small Language menu so people can override the system language without changing macOS settings.

Two commits:

1. feat(i18n): localize the interface with a String Catalog (zh-Hans) — adds Compositor/Localizable.xcstrings (source language en, zh-Hans added to knownRegions) and routes user-visible strings through it.
2. feat(i18n): add a Language menu (System / English / 简体中文) — LanguageSettings stores the override in UserDefaults; the panel reuses the existing FloatingPanelController.

Coverage

• Menus — File/Edit/View/Select/Image/Filter/Layer items. The dynamic filter and adjustment names are localized through a new localizedName on FilterKind/AdjustmentKind (English raw value stays the catalog key).
• Tool rail — every tool's name plus the status-bar hint. That hint was one giant nested ternary, which SwiftUI type-infers as String and therefore never localizes; it is now a switch of LocalizedStringKey literals (toolHintMessage).
• Inspectors — transform, brush, gradient, type, shape, marquee/lasso/wand, levels, hue/saturation, curves, effects, Camera Raw, image/canvas size, trim.
• Enum pickers — every segmented control that rendered a raw value (sampling, selection mode, blend modes, Camera Raw pages, units, alignment, …) now shows a localized name.
• Dialogs — the unsaved-changes NSAlert (AppKit strings don't flow through SwiftUI's catalog, so they use String(localized:), with %@ for the document name).

Notes for review

• extractionState is "legacy" in the catalog. Symbol generation can't produce valid Swift identifiers for keys like %, Type, or the Feather / Feather… pair, and legacy opts those keys out of symbol generation while keeping runtime lookup intact. Build logs before that change were full of This key is too close to a reserved keyword / would generate the same symbol.
• Chinese terminology follows Simplified Chinese Photoshop (图层, 蒙版, 羽化, 色阶, 曲线, 内容感知填充, …), so the UI reads the way PS users expect.
• Untranslated keys remain "new" in the catalog for strings that are intentionally left alone (file magic like 8BPS, internal identifiers).

Testing

• xcodebuild -project Compositor.xcodeproj -scheme Compositor -configuration Debug build succeeds (Xcode 26.1.1, ad-hoc signing).
• Ran the app with -AppleLanguages (zh-Hans): menus, tool rail, status bar, and the sheets listed above render in Chinese; switching the Language menu to English and back changes the interface after relaunch.
• Verified the compiled zh-Hans.lproj/Localizable.strings in the built bundle contains the expected keys.

[MJorgin]

Quick context: I put this together while setting Compositor up for Simplified Chinese users, and kept the terminology aligned with Simplified Chinese Photoshop (图层 / 蒙版 / 羽化 / 色阶 / 曲线 / 内容感知填充) so it reads naturally to anyone coming from PS.

Happy to change anything you would prefer — if you would rather merge only the translations and drop the in-app Language menu, I can split that out. Same for wording, terminology, or file layout.

The catalog uses extractionState "legacy" deliberately: keys like % , Type, and the Feather / Feather… pair cannot generate valid Swift symbols, and legacy opts them out of symbol generation while keeping runtime lookup working.

Thanks for open-sourcing the app.

[robbietilton]

Thanks for putting this together, and sorry to turn it down.

Localization is on hold for now, for the same reasons I gave on #57: Compositor is new and it's just me maintaining it. Localization isn't a one-time contribution, since every string I write from then on needs translating, and I can't review translations in languages I don't read, which would mean shipping text I can't verify.

I'll revisit it once things settle, and when I do, a String Catalog is genuinely the useful groundwork. Closing for now.