fix(i18n): resolve plugin-pack plurals with the pack's declared locale (#12505)

Plugin language packs register under synthetic plugin<hex> resource
languages, so i18next's plural resolver fell back to English rules and
Russian/Polish/etc. packs never selected their _few/_many forms. Map each
pack's resource language to its declared (Intl-supported) locale inside
the plural resolver, and fill missing plural categories from the key's
_other form so packs that only ship _one/_other stay in-language.

Co-authored-by: Neil <neil@stably.ai>

src/renderer/src/i18n/i18n.ts

@@ -18,7 +18,10 @@ import { advanceTranslationResourceVersion } from './translation-resource-versio
 import { DEFAULT_LOCALE, resolveUiLocale } from './supported-languages'
 import type { SupportedUiLocale } from '../../../shared/ui-locale'
 import { isPluginUiLanguage, type UiLanguage } from '../../../shared/ui-language'
-import type { PluginLanguagePackRegistration } from '../../../shared/plugins/plugin-language-pack-artifact'
+import {
+  isCatalogObject,
+  type PluginLanguagePackRegistration
+} from '../../../shared/plugins/plugin-language-pack-artifact'
 import { withTimeout } from '../../../shared/promise-timeout-fallback'
 
 export const i18n: I18nInstance = i18next.createInstance()
@@ -88,6 +91,13 @@ void i18n
 i18n.store.on('added', advanceTranslationResourceVersion)
 i18n.store.on('removed', advanceTranslationResourceVersion)
 
+// Synthetic plugin<hex> tags would get English plurals; plugin-pack-plurals.test.ts guards i18next upgrades.
+const pluralLocaleByResourceLanguage = new Map<string, string>()
+const pluralResolver = i18n.services.pluralResolver
+const getPluralRule = pluralResolver.getRule.bind(pluralResolver)
+pluralResolver.getRule = (code: string, options?: { ordinal?: boolean }) =>
+  getPluralRule(pluralLocaleByResourceLanguage.get(code) ?? code, options)
+
 export function translate(key: string, fallback: string, options?: TOptions): string {
   const value = i18n.t(key, { defaultValue: fallback, ...options })
   return isPseudoLocalizationLocale(i18n.language) ? pseudoLocalizeString(value) : value
@@ -125,10 +135,16 @@ let pluginLanguagePacks: readonly PluginLanguagePackRegistration[] = []
 export function getIntlLocale(): string {
   const active = i18n.language
   const pack = pluginLanguagePacks.find((entry) => entry.resourceLanguage === active)
-  const candidate = pack?.locale ?? active
+  return supportedIntlLocale(pack?.locale ?? active, Intl.DateTimeFormat)
+}
+
+function supportedIntlLocale(
+  locale: string,
+  intl: { supportedLocalesOf(locale: string): string[] }
+): string {
   try {
     // An unsupported tag would silently use the runtime locale; explicitly fall back to the default.
-    return Intl.DateTimeFormat.supportedLocalesOf(candidate)[0] ?? DEFAULT_LOCALE
+    return intl.supportedLocalesOf(locale)[0] ?? DEFAULT_LOCALE
   } catch {
     return DEFAULT_LOCALE
   }
@@ -138,6 +154,24 @@ export function resolveRendererResourceLanguage(language: string): string {
   return pluginLanguagePacks.find((pack) => pack.id === language)?.resourceLanguage ?? language
 }
 
+// Packs translated from English often ship only _one/_other; reuse _other so few/many stay in-language.
+function withPluralCategoriesFromOther(
+  catalog: Record<string, unknown>,
+  categories: readonly string[]
+): Record<string, unknown> {
+  const filled: Record<string, unknown> = {}
+  for (const [key, value] of Object.entries(catalog)) {
+    filled[key] = isCatalogObject(value) ? withPluralCategoriesFromOther(value, categories) : value
+    if (typeof value === 'string' && key.endsWith('_other')) {
+      const base = key.slice(0, -'_other'.length)
+      for (const category of categories) {
+        filled[`${base}_${category}`] ??= value
+      }
+    }
+  }
+  return filled
+}
+
 export function setRendererPluginLanguagePacks(
   packs: readonly PluginLanguagePackRegistration[]
 ): void {
@@ -145,9 +179,19 @@ export function setRendererPluginLanguagePacks(
     i18n.removeResourceBundle(language, 'translation')
   }
   registeredPluginLanguages.clear()
+  pluralLocaleByResourceLanguage.clear()
   pluginLanguagePacks = packs
   for (const pack of packs) {
-    i18n.addResourceBundle(pack.resourceLanguage, 'translation', pack.catalog, true, true)
+    const pluralLocale = supportedIntlLocale(pack.locale, Intl.PluralRules)
+    pluralLocaleByResourceLanguage.set(pack.resourceLanguage, pluralLocale)
+    const categories = new Intl.PluralRules(pluralLocale).resolvedOptions().pluralCategories
+    i18n.addResourceBundle(
+      pack.resourceLanguage,
+      'translation',
+      withPluralCategoriesFromOther(pack.catalog, categories),
+      true,
+      true
+    )
     registeredPluginLanguages.add(pack.resourceLanguage)
   }
 }

src/renderer/src/i18n/plugin-pack-plurals.test.ts

@@ -0,0 +1,45 @@
+import { afterEach, describe, expect, it } from 'vitest'
+import { i18n, setRendererPluginLanguagePacks } from './i18n'
+import { pluginLanguageResourceId } from '../../../shared/plugins/plugin-language-pack-artifact'
+import { DEFAULT_LOCALE } from './supported-languages'
+
+const PACK_ID = 'plugin:smwbev.russian/ru-RU' as const
+const PACK_RESOURCE = pluginLanguageResourceId(PACK_ID)
+const CATALOG = { files_one: 'one', files_few: 'few', files_many: 'many', files_other: 'other' }
+
+async function activatePack(
+  locale: string,
+  catalog: Record<string, string> = CATALOG
+): Promise<void> {
+  setRendererPluginLanguagePacks([
+    {
+      id: PACK_ID,
+      resourceLanguage: PACK_RESOURCE,
+      pluginKey: 'smwbev.russian',
+      locale,
+      catalog
+    }
+  ])
+  await i18n.changeLanguage(PACK_RESOURCE)
+}
+
+function forms(counts: number[]): string[] {
+  return counts.map((count) => i18n.t('files', { count }))
+}
+
+afterEach(async () => {
+  setRendererPluginLanguagePacks([])
+  await i18n.changeLanguage(DEFAULT_LOCALE)
+})
+
+describe('plugin pack plurals', () => {
+  it('selects plural forms by the locale the pack declares', async () => {
+    await activatePack('ru-RU')
+    expect(forms([1, 2, 5, 21])).toEqual(['one', 'few', 'many', 'one'])
+  })
+
+  it('keeps packs with only _one/_other in-language for few/many counts', async () => {
+    await activatePack('ru-RU', { files_one: 'one', files_other: 'other' })
+    expect(forms([1, 2, 5, 21])).toEqual(['one', 'other', 'other', 'one'])
+  })
+})

src/shared/plugins/plugin-language-pack-artifact.ts

@@ -66,7 +66,7 @@ type CatalogFrame = {
   depth: number
 }
 
-function isCatalogObject(value: unknown): value is Record<string, unknown> {
+export function isCatalogObject(value: unknown): value is Record<string, unknown> {
   return (
     typeof value === 'object' &&
     value !== null &&