diff --git a/src/shared/plugins/plugin-content-pack-contributions.test.ts b/src/shared/plugins/plugin-content-pack-contributions.test.ts
index b3d1708ee02..7b64f9a3f1c 100644
--- a/src/shared/plugins/plugin-content-pack-contributions.test.ts
+++ b/src/shared/plugins/plugin-content-pack-contributions.test.ts
@@ -120,6 +120,9 @@ describe('content-pack manifest contributions', () => {
 
   it.each([
     ['language pack', { languagePacks: [{ locale: 'en_US', path: 'locale.json' }] }],
+    // Shape-legal but ill-formed: the regex alone let these load with dates falling back to English.
+    ['duplicate region', { languagePacks: [{ locale: 'en-US-US', path: 'locale.json' }] }],
+    ['unknown subtag', { languagePacks: [{ locale: 'en-aaa', path: 'locale.json' }] }],
     ['language pack path', { languagePacks: [{ locale: 'pt-BR', path: '../outside.json' }] }],
     ['VM recipe', { vmRecipes: [{ path: '\\\\server\\recipe.json' }] }],
     ['agent profile', { agents: [{ path: 'agents/../profile.json' }] }]
@@ -127,6 +130,14 @@ describe('content-pack manifest contributions', () => {
     expect(parsePluginManifest(manifest(contributes)).ok).toBe(false)
   })
 
+  it.each(['ru', 'pt-BR', 'zh-Hans-CN', 'en-US'])('accepts the well-formed locale %s', (locale) => {
+    expect(
+      parsePluginManifest(manifest({ languagePacks: [{ locale, path: 'locale.json' }] }))
+    ).toMatchObject({
+      ok: true
+    })
+  })
+
   it('rejects duplicate ids, locales, paths, and bindings', () => {
     const parsed = pluginManifestSchema.safeParse(
       manifest({
diff --git a/src/shared/plugins/plugin-content-pack-contributions.ts b/src/shared/plugins/plugin-content-pack-contributions.ts
index 2ed599d11ba..e8e691ee12e 100644
--- a/src/shared/plugins/plugin-content-pack-contributions.ts
+++ b/src/shared/plugins/plugin-content-pack-contributions.ts
@@ -7,12 +7,24 @@ export const PLUGIN_KEYBINDING_LIMIT = 256
 export const PLUGIN_VM_RECIPE_LIMIT = 64
 export const PLUGIN_AGENT_PROFILE_LIMIT = 64
 
+/** True when Intl can parse the tag's grammar, not just its shape. */
+function isWellFormedLocaleId(value: string): boolean {
+  try {
+    Intl.getCanonicalLocales(value)
+    return true
+  } catch {
+    return false
+  }
+}
+
 // Locale IDs become filenames/i18next keys; a bounded BCP 47 subset excludes path syntax.
+// Intl rejects shape-valid, ill-formed tags (`en-US-US`) at install, not silently when formatting.
 export const pluginLocaleIdSchema = z
   .string()
   .min(2)
   .max(35)
   .regex(/^[A-Za-z]{2,3}(?:-[A-Za-z0-9]{2,8})*$/, 'must be a portable locale identifier')
+  .refine(isWellFormedLocaleId, 'must be a well-formed locale identifier')
 
 export const pluginLanguagePackContributionSchema = z
   .object({