diff --git a/packages/docusaurus-theme-translations/locales/__tests__/locales.test.ts b/packages/docusaurus-theme-translations/locales/__tests__/locales.test.ts
index 16299fb1997e..9b899b4fa248 100644
--- a/packages/docusaurus-theme-translations/locales/__tests__/locales.test.ts
+++ b/packages/docusaurus-theme-translations/locales/__tests__/locales.test.ts
@@ -42,4 +42,66 @@ describe('theme translations', () => {
 
     expect(codeMessages).toEqual(baseMessages);
   });
+
+  // The update script only logs "Some localized messages do not exist in
+  // base.json!" and never fails. When a message id is renamed in the theme
+  // code, the translation stays under the old id that nothing reads anymore,
+  // and the new id falls back to the English base text.
+  it('has localized messages files that only use the message ids of the base messages files', async () => {
+    const staleMessages = await getStaleLocalizedMessages();
+
+    expect(
+      staleMessages,
+      'These localized message ids do not exist in the base messages files. Move their translation to the new message id if it was renamed, or delete them',
+    ).toEqual([]);
+  });
 });
+
+const LocalesDirPath = path.join(__dirname, '..');
+
+type Messages = {[id: string]: string};
+
+// Returns the "<locale>/<file>: <id>" localized messages whose id is not in
+// the matching base messages file
+async function getStaleLocalizedMessages(): Promise<string[]> {
+  const files = await fs.readdir(path.join(LocalesDirPath, 'base'));
+  const locales = await getLocales();
+
+  const staleMessages = await Promise.all(
+    locales.flatMap((locale) =>
+      files.map((file) => getStaleLocaleFileMessages(locale, file)),
+    ),
+  );
+  return staleMessages.flat();
+}
+
+async function getStaleLocaleFileMessages(
+  locale: string,
+  file: string,
+): Promise<string[]> {
+  const baseMessages = await readMessagesFile('base', file);
+  const localeMessages = await readMessagesFile(locale, file);
+
+  const staleIds = _.difference(
+    Object.keys(localeMessages),
+    Object.keys(baseMessages),
+  );
+  return staleIds.map((id) => `${locale}/${file}: ${id}`);
+}
+
+async function getLocales(): Promise<string[]> {
+  const entries = await fs.readdir(LocalesDirPath, {withFileTypes: true});
+  const localeDirs = entries.filter(
+    (entry) =>
+      entry.isDirectory() &&
+      // Ignore hidden directories created by tools
+      !entry.name.startsWith('.') &&
+      entry.name !== 'base' &&
+      entry.name !== '__tests__',
+  );
+  return localeDirs.map((entry) => entry.name);
+}
+
+function readMessagesFile(locale: string, file: string): Promise<Messages> {
+  return readJSON(path.join(LocalesDirPath, locale, file)) as Promise<Messages>;
+}
diff --git a/packages/docusaurus-theme-translations/locales/is/theme-common.json b/packages/docusaurus-theme-translations/locales/is/theme-common.json
index 7ec1701aa1e0..05e5a3dd8258 100644
--- a/packages/docusaurus-theme-translations/locales/is/theme-common.json
+++ b/packages/docusaurus-theme-translations/locales/is/theme-common.json
@@ -49,8 +49,7 @@
   "theme.contentVisibility.draftBanner.title": "Draft page",
   "theme.contentVisibility.unlistedBanner.message": "Þessi síða er ólistuð. Leitarvélar munu ekki skrá hana, eingöngu notendur með beinan hlekk geta opnað hana.",
   "theme.contentVisibility.unlistedBanner.title": "Óskráð síða",
-  "theme.docs.DocCard.categoryDescription": "{count} atriði",
-  "theme.docs.DocCard.categoryDescription.plurals": "1 item|{count} items",
+  "theme.docs.DocCard.categoryDescription.plurals": "1 atriði|{count} atriði",
   "theme.docs.breadcrumbs.home": "Heimasíða",
   "theme.docs.breadcrumbs.navAriaLabel": "Brauðteningar",
   "theme.docs.paginator.navAriaLabel": "Skjala síður",