@@ -0,0 +1,102 @@
+import { describe, expect, it } from 'vitest'
+import fs from 'fs'
+import path from 'path'
+import { getSupportedLanguages } from '../../../src/common/i18n'
+import { getLanguageOptions } from '../../../src/renderer/src/prefComponents/general/config'
+import { MUYA_LOCALES } from '../../../src/renderer/src/util/muyaLocale'
+
+/**
+ * Adding a UI language means editing several lists that nothing else ties
+ * together, and they have drifted twice: `nl` reached the desktop but never the
+ * engine locale map (#5499, English editor UI for Dutch users), and `ru` reached
+ * both but not `electronLanguages` (#5496), which strips the Chromium/AppKit
+ * Russian resources out of packaged builds — the regression #5132 fixed.
+ *
+ * Each `it` below names the file to edit when it fails.
+ */
+
+const DESKTOP_ROOT = path.join(__dirname, '../../..')
+const LOCALES_DIR = path.join(DESKTOP_ROOT, 'static/locales')
+const BUILDER_CONFIG = path.join(DESKTOP_ROOT, 'electron-builder.yml')
+
+const localeFileTags = (): string[] =>
+  fs
+    .readdirSync(LOCALES_DIR)
+    .filter(f => f.endsWith('.json') && !f.endsWith('.min.json'))
+    .map(f => f.replace('.json', ''))
+
+/**
+ * Reads the `electronLanguages:` block out of electron-builder.yml.
+ *
+ * Hand-parsed rather than via js-yaml: no YAML parser is a declared dependency
+ * of this package, and picking up the copy electron-builder hoists into
+ * node_modules would be a phantom dependency (the failure mode of #5159).
+ * The block is a flat list of scalars, so recognising `  - value` lines and
+ * skipping comments is enough.
+ */
+const readElectronLanguages = (): string[] => {
+  const lines = fs.readFileSync(BUILDER_CONFIG, 'utf8').split('\n')
+  const start = lines.findIndex(line => line.startsWith('electronLanguages:'))
+  if (start === -1) return []
+
+  const values: string[] = []
+  for (const line of lines.slice(start + 1)) {
+    if (/^\s*#/.test(line) || line.trim() === '') continue
+    const entry = /^\s+-\s+(\S+)\s*$/.exec(line)
+    if (!entry) break
+    values.push(entry[1]!)
+  }
+  return values
+}
+
+// macOS .lproj bundles spell the region with an underscore, Windows/Linux .pak
+// files with a hyphen; electron-builder.yml carries both spellings.
+const primarySubtag = (tag: string): string => tag.replace('_', '-').split('-')[0]!.toLowerCase()
+
+describe('language registries agree', () => {
+  const supported = getSupportedLanguages()
+
+  it('has a non-empty set of supported languages to check', () => {
+    expect(supported.length).toBeGreaterThan(0)
+  })
+
+  it('ships a locale file for every supported language (static/locales/<tag>.json)', () => {
+    expect([...localeFileTags()].sort()).toEqual([...supported].sort())
+  })
+
+  it('offers every supported language in Preferences (prefComponents/general/config.ts)', () => {
+    const offered = getLanguageOptions().map(option => option.value)
+    expect([...offered].sort()).toEqual([...supported].sort())
+  })
+
+  it('maps every supported language to an engine locale (util/muyaLocale.ts)', () => {
+    expect(Object.keys(MUYA_LOCALES).sort()).toEqual([...supported].sort())
+  })
+
+  describe('packaged Electron locale resources (electron-builder.yml)', () => {
+    const electronLanguages = readElectronLanguages()
+
+    it('still finds the electronLanguages block', () => {
+      expect(electronLanguages.length).toBeGreaterThan(0)
+    })
+
+    it('keeps a resource for every supported language', () => {
+      // `pt` has no bare entry — Electron ships pt-BR/pt-PT — so match on the
+      // primary subtag rather than requiring the exact tag.
+      const covered = new Set(electronLanguages.map(primarySubtag))
+      const missing = supported.filter(tag => !covered.has(primarySubtag(tag)))
+      expect(
+        missing,
+        '\n  Add these to electronLanguages in electron-builder.yml, or the packaged' +
+          '\n  app drops their .pak/.lproj resources and OS-provided UI stays English:' +
+          `\n    ${missing.join(', ')}\n`
+      ).toEqual([])
+    })
+
+    it('keeps no resource for a language that was dropped', () => {
+      const supportedPrimaries = new Set(supported.map(primarySubtag))
+      const stale = electronLanguages.filter(tag => !supportedPrimaries.has(primarySubtag(tag)))
+      expect(stale, `\n  electronLanguages entries with no matching UI language: ${stale.join(', ')}\n`).toEqual([])
+    })
+  })
+})