Track language selection with Plausible Language Changed event.

Centralize UI language switches in cycleAppLanguage and document the event in plausible-events.md.

Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/utils/i18nLanguages.test.ts

@@ -1,7 +1,40 @@
-import { describe, expect, it } from 'vitest'
-import { getNextLanguage, normalizeAppLanguage, SUPPORTED_LANGUAGES } from './i18nLanguages.js'
+import type { i18n as I18nInstance } from 'i18next'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { PlausibleEvents } from '../services/analytics.js'
+import {
+  changeAppLanguage,
+  cycleAppLanguage,
+  getNextLanguage,
+  normalizeAppLanguage,
+  SUPPORTED_LANGUAGES
+} from './i18nLanguages.js'
+
+const trackPlausibleEvent = vi.fn()
+
+vi.mock('../services/analytics.js', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('../services/analytics.js')>()
+  return {
+    ...actual,
+    trackPlausibleEvent: (...args: unknown[]) => trackPlausibleEvent(...args)
+  }
+})
+
+function createMockI18n(language: string): I18nInstance {
+  let current = language
+  return {
+    language: current,
+    changeLanguage: vi.fn(async (lng: string) => {
+      current = lng
+      ;(this as { language: string }).language = lng
+    })
+  } as unknown as I18nInstance
+}
 
 describe('i18nLanguages', () => {
+  beforeEach(() => {
+    trackPlausibleEvent.mockReset()
+  })
+
   it('normalizes regional tags to supported base codes', () => {
     expect(normalizeAppLanguage('de-DE')).toBe('de')
     expect(normalizeAppLanguage('nb-NO')).toBe('nb')
@@ -18,4 +51,33 @@ describe('i18nLanguages', () => {
     expect(seen.size).toBe(SUPPORTED_LANGUAGES.length)
     expect(current).toBe('de')
   })
+
+  it('tracks explicit language changes', () => {
+    const i18n = createMockI18n('de')
+    changeAppLanguage(i18n, 'sv')
+
+    expect(i18n.changeLanguage).toHaveBeenCalledWith('sv')
+    expect(trackPlausibleEvent).toHaveBeenCalledWith(PlausibleEvents.LANGUAGE_CHANGED, {
+      from: 'de',
+      to: 'sv'
+    })
+  })
+
+  it('does not track when language stays the same', () => {
+    const i18n = createMockI18n('en')
+    changeAppLanguage(i18n, 'en')
+
+    expect(i18n.changeLanguage).not.toHaveBeenCalled()
+    expect(trackPlausibleEvent).not.toHaveBeenCalled()
+  })
+
+  it('cycleAppLanguage tracks the next language', () => {
+    const i18n = createMockI18n('nb')
+    cycleAppLanguage(i18n)
+
+    expect(trackPlausibleEvent).toHaveBeenCalledWith(PlausibleEvents.LANGUAGE_CHANGED, {
+      from: 'nb',
+      to: 'de'
+    })
+  })
 })