diff --git a/src/renderer/src/components/stats/StatsPane.tsx b/src/renderer/src/components/stats/StatsPane.tsx
index 002a2dca4ae7..4d62ad89bb59 100644
--- a/src/renderer/src/components/stats/StatsPane.tsx
+++ b/src/renderer/src/components/stats/StatsPane.tsx
@@ -15,7 +15,7 @@ import {
   DropdownMenuTrigger
 } from '../ui/dropdown-menu'
 import { AgentIcon } from '@/lib/agent-catalog'
-import { translate } from '@/i18n/i18n'
+import { getIntlLocale, translate } from '@/i18n/i18n'
 export { getStatsPaneSearchEntries } from './stats-search'
 
 function formatDuration(ms: number): string {
@@ -44,7 +44,7 @@ function formatTrackingSince(timestamp: number | null): string {
   }
   const date = new Date(timestamp)
   return translate('auto.components.stats.StatsPane.trackingSince', 'Tracking since {{value0}}', {
-    value0: date.toLocaleDateString(undefined, {
+    value0: date.toLocaleDateString(getIntlLocale(), {
       month: 'short',
       day: 'numeric',
       year: 'numeric'
diff --git a/src/renderer/src/i18n/i18n.ts b/src/renderer/src/i18n/i18n.ts
index 3f35e2ec2244..0d5cbad54cfa 100644
--- a/src/renderer/src/i18n/i18n.ts
+++ b/src/renderer/src/i18n/i18n.ts
@@ -95,6 +95,24 @@ export async function setRendererUiLanguage(language: UiLanguage): Promise<void>
 const registeredPluginLanguages = new Set<string>()
 let pluginLanguagePacks: readonly PluginLanguagePackRegistration[] = []
 
+/**
+ * BCP-47 tag for `Intl` formatting. Plugin catalogs register under a synthetic
+ * `plugin<hex>` resource language that `Intl` rejects, so fall back to the tag
+ * the pack declares (for example `ru-RU`) and finally to the default locale.
+ */
+export function getIntlLocale(): string {
+  const active = i18n.language
+  const pack = pluginLanguagePacks.find((entry) => entry.resourceLanguage === active)
+  const candidate = pack?.locale ?? active
+  try {
+    // Why: an empty result means Intl has no data for the tag, so fall through
+    // to the default locale instead of letting Intl pick the runtime one.
+    return Intl.DateTimeFormat.supportedLocalesOf(candidate)[0] ?? DEFAULT_LOCALE
+  } catch {
+    return DEFAULT_LOCALE
+  }
+}
+
 export function resolveRendererResourceLanguage(language: string): string {
   return pluginLanguagePacks.find((pack) => pack.id === language)?.resourceLanguage ?? language
 }
diff --git a/src/renderer/src/i18n/intl-locale.test.ts b/src/renderer/src/i18n/intl-locale.test.ts
new file mode 100644
index 000000000000..69f0de084555
--- /dev/null
+++ b/src/renderer/src/i18n/intl-locale.test.ts
@@ -0,0 +1,79 @@
+/**
+ * `getIntlLocale()` exists because plugin catalogs register under a synthetic
+ * `plugin<hex>` resource language. Passing that straight to `Intl` throws, and
+ * passing `undefined` silently falls back to the OS locale rather than the
+ * language the user selected.
+ *
+ * The branch assertions stub `supportedLocalesOf` so they describe the helper's
+ * logic rather than whichever locales the runtime's ICU build happens to carry.
+ */
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { getIntlLocale, 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 PACK = {
+  id: PACK_ID,
+  resourceLanguage: PACK_RESOURCE,
+  pluginKey: 'smwbev.russian',
+  locale: 'ru-RU',
+  catalog: {}
+}
+
+type Packs = Parameters<typeof setRendererPluginLanguagePacks>[0]
+
+async function activate(language: string, packs: Packs = []): Promise<void> {
+  setRendererPluginLanguagePacks(packs)
+  await i18n.changeLanguage(language)
+}
+
+/** Stubs ICU lookup so a locale counts as supported only when listed. */
+function withSupportedLocales(supported: readonly string[]): void {
+  vi.spyOn(Intl.DateTimeFormat, 'supportedLocalesOf').mockImplementation((requested) => {
+    const tags = Array.isArray(requested) ? requested : [requested as string]
+    return tags.filter((tag) => supported.includes(tag)) as string[]
+  })
+}
+
+afterEach(async () => {
+  vi.restoreAllMocks()
+  setRendererPluginLanguagePacks([])
+  await i18n.changeLanguage(DEFAULT_LOCALE)
+})
+
+describe('getIntlLocale', () => {
+  it('passes a supported built-in locale straight through', async () => {
+    withSupportedLocales(['es'])
+    await activate('es')
+    expect(getIntlLocale()).toBe('es')
+  })
+
+  it('resolves a plugin resource language to the locale the pack declares', async () => {
+    withSupportedLocales(['ru-RU'])
+    await activate(PACK_RESOURCE, [PACK])
+    // Without the pack lookup this would reach Intl as `plugin<hex>`.
+    expect(getIntlLocale()).toBe('ru-RU')
+  })
+
+  it('falls back to the default locale when ICU has no data for the tag', async () => {
+    withSupportedLocales([])
+    await activate('es')
+    // Returning the tag here would let Intl silently format with the runtime locale.
+    expect(getIntlLocale()).toBe(DEFAULT_LOCALE)
+  })
+
+  it('falls back to the default locale when Intl rejects the tag', async () => {
+    vi.spyOn(Intl.DateTimeFormat, 'supportedLocalesOf').mockImplementation(() => {
+      throw new RangeError('invalid language tag')
+    })
+    await activate(PACK_RESOURCE)
+    expect(getIntlLocale()).toBe(DEFAULT_LOCALE)
+  })
+
+  it('keeps the synthetic resource language unusable for Intl directly', () => {
+    // Unstubbed on purpose: this is a property of the tag, not of ICU data.
+    expect(() => Intl.DateTimeFormat.supportedLocalesOf(PACK_RESOURCE)).toThrow(RangeError)
+  })
+})