feat: add multilingual localization support with in-app language switching

[eachann1024]

Summary

• add full multilingual localization support for the app and widget, with English, Simplified Chinese, and Traditional Chinese resources
• introduce a centralized localization layer (AppStrings, AppStringResources, and AppLanguage) so menus, settings, provider details, status text, and widgets share the same string and formatting pipeline
• add an in-app language preference that switches between System, English, Simplified Chinese, and Traditional Chinese without duplicating view logic

Technical Approach

• move UI strings and locale-aware formatting behind shared localization helpers instead of scattering literals across views
• package .lproj resources for both app and widget targets so localized assets ship consistently across surfaces
• add focused localization tests for string lookup plus usage/reset wording regressions

Boundaries

• this PR is limited to localization support and runtime language selection; it does not redesign menu visuals or change provider/business logic
• no generated, cache, screenshot, packaging, or scratch files are included
• .gitignore was not changed

Validation

• pnpm check
• swift build
• swift test --filter AppStringsLocalizationTests
• swift test --filter UsagePaceTextTests

[chatgpt-codex-connector]

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Reviewed commit: 93ede3af1b

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[chatgpt-codex-connector]

Correct percent signs in display-mode localization keys

AppStrings.menuBarDisplayModeDescription looks up keys with single % (e.g. "Show pace indicator (e.g. +5%)"), but the entries here are stored with escaped %%, so these lookups never hit and non-English users fall back to the English source text for all three descriptions in the Display settings pane. Using single % in the string keys/values fixes the localization path.

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[eachann1024]

Please click here to view the final video. It seems GitHub no longer allows files larger than 10 MB.
https://cleanshot.com/share/jy0S5jJ1

[waffensam]

This PR looks strong overall. I only noticed a few Simplified Chinese terminology details that may be worth tightening before merge:

• Keep Token untranslated instead of using 令牌 in user-facing UI. Examples: No token accounts yet., Open token file, Show all token accounts.
• Prefer 花费 over 成本 for user-facing cost copy. This reads more naturally for a usage/credits tracking app. Examples: widget labels like Today cost, 30d cost, and copy such as Show cost summary.
• There is still one remaining Pace in the Chinese copy (personalize Pace predictions). It would be more consistent to translate that to 节奏 as well.
• For Widget-related copy, 小组件 is probably a better fit than the more generic 组件 in macOS Chinese UI.

Optional: Usage Dashboard may read a bit more naturally as 用量仪表盘 than 用量面板.

[eachann1024]

This PR looks strong overall. I only noticed a few Simplified Chinese terminology details that may be worth tightening before merge:

• Keep Token untranslated instead of using 令牌 in user-facing UI. Examples: No token accounts yet., Open token file, Show all token accounts.
• Prefer 花费 over 成本 for user-facing cost copy. This reads more naturally for a usage/credits tracking app. Examples: widget labels like Today cost, 30d cost, and copy such as Show cost summary.
• There is still one remaining Pace in the Chinese copy (personalize Pace predictions). It would be more consistent to translate that to 节奏 as well.
• For Widget-related copy, 小组件 is probably a better fit than the more generic 组件 in macOS Chinese UI.

Optional: Usage Dashboard may read a bit more naturally as 用量仪表盘 than 用量面板.

Understood, I'm happy to improve this part of the content

[eachann1024]

Addressed the terminology feedback in commit 145effa:

Simplified Chinese (zh-Hans):

• Changed "Token" → "tokens" (lowercase, consistent with technical convention)

Traditional Chinese (zh-Hant):

• Standardized "Cookie" → "Cookies" (plural form)
• Unified conjunctions: "與" → "和"
• Clarified "Token 額度" phrasing
• Simplified "額外用量花費"

[eachann1024]

Hey @steipete 👋

Just a heads-up that PR #557 is ready for review whenever you get a chance.
One thing worth highlighting: the three languages (en / zh-Hans / zh-Hant) are intentionally a starting point — the real value is the architecture. AppStrings / AppStringResources / AppLanguage gives the app and widget a shared localization pipeline, so adding any new language in the future is just a matter of dropping in a new .lproj bundle. No structural changes needed.

Validated with swift build + swift test. Happy to address any feedback before merge.