diff --git a/CHANGELOG.md b/CHANGELOG.md
index 62310e8..b427d70 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -23,6 +23,11 @@ history stays consistent across GitHub and npm.
   and `routable`, JWT or API-key auth, and checks SEO and media alt text. Added
   `cms-lab init --cms payload`.
 
+- Added opt-in localization completeness via `checks.localization`. Groups
+  published documents by a translation key (`groupField`, default
+  `document.uid`) and emits `CMS-LOCALE-MISSING` (warning) for any group that
+  is not published in every configured locale. Adds a `localization` check
+  group for `--only` / `--skip`.
 - Added `CMS-RELATIONSHIP-UNPUBLISHED` (warning): a published document whose
   relationship rule is satisfied only by draft records links to nothing live at
   runtime. Relationship checks are now status-aware.
diff --git a/apps/site/app/docs/configuration/page.tsx b/apps/site/app/docs/configuration/page.tsx
index aeaf784..34f2cbf 100644
--- a/apps/site/app/docs/configuration/page.tsx
+++ b/apps/site/app/docs/configuration/page.tsx
@@ -20,6 +20,7 @@ export default function ConfigurationPage() {
         { href: "#route-fields", label: "Route fields" },
         { href: "#required-fields", label: "Required fields" },
         { href: "#relationships", label: "Relationships" },
+        { href: "#localization", label: "Localization" },
         { href: "#custom-rules", label: "Custom rules" },
       ]}
     >
@@ -390,6 +391,31 @@ export default defineConfig({
         <code>CMS-RELATIONSHIP-UNPUBLISHED</code>, since the live page links to
         nothing live at runtime.
       </div>
+
+      <h2 id="localization">Localization</h2>
+      <p>
+        Localization checks flag locale-parity gaps: a content group that is
+        published in some locales but missing in others. Documents are grouped
+        by <code>groupField</code> (defaulting to the document UID, common when
+        locales share a slug), and only published documents count, so a
+        translation that is still a draft is reported as missing.
+      </p>
+      <CodeBlock>{`checks: {
+  localization: {
+    locales: ["en", "fr", "de"],
+    // localeField defaults to "locale"; groupField defaults to the document UID.
+    localeField: "locale",
+    groupField: "translationKey",
+    types: ["page", "article"],
+    severity: "warning",
+  },
+}`}</CodeBlock>
+      <div className="callout">
+        <strong>Opt-in</strong>
+        The check runs only when <code>checks.localization</code> is set, and
+        emits <code>CMS-LOCALE-MISSING</code> for each group missing a locale.
+        Set <code>types</code> to limit it to localized content types.
+      </div>
       <div className="callout">
         <strong>Provider fields</strong>
         SEO and image checks understand the common native shapes from the
diff --git a/apps/site/app/docs/diagnostics/page.tsx b/apps/site/app/docs/diagnostics/page.tsx
index 82b8cbd..0a1f48d 100644
--- a/apps/site/app/docs/diagnostics/page.tsx
+++ b/apps/site/app/docs/diagnostics/page.tsx
@@ -108,6 +108,14 @@ export default function DiagnosticsPage() {
             A published document&apos;s relationship is satisfied only by draft
             records, so it links to nothing live at runtime.
           </DiagnosticCode>
+          <DiagnosticCode
+            code="CMS-LOCALE-MISSING"
+            severity="warning"
+            group="localization"
+          >
+            A content group has no published document in one or more configured
+            locales. Opt-in via <code>checks.localization</code>.
+          </DiagnosticCode>
           <DiagnosticCode
             code="SEO-META-MISSING"
             severity="warning"
diff --git a/packages/cli/src/exporters.ts b/packages/cli/src/exporters.ts
index d3f9939..c2c3a84 100644
--- a/packages/cli/src/exporters.ts
+++ b/packages/cli/src/exporters.ts
@@ -134,6 +134,10 @@ function groupForDiagnostic(diagnostic: Diagnostic): string {
     return "relationships";
   }
 
+  if (diagnostic.code.startsWith("CMS-LOCALE")) {
+    return "localization";
+  }
+
   if (diagnostic.code.startsWith("SEO-")) {
     return "seo";
   }
diff --git a/packages/cli/src/index.ts b/packages/cli/src/index.ts
index 9db04c8..ae0173e 100644
--- a/packages/cli/src/index.ts
+++ b/packages/cli/src/index.ts
@@ -1688,6 +1688,7 @@ function parseCheckGroups(values: string[] | undefined): CheckGroup[] {
     "fields",
     "relationships",
     "custom",
+    "localization",
   ]);
   const groups = splitList(values);
 
diff --git a/packages/core/src/config.test.ts b/packages/core/src/config.test.ts
index c9d1ed4..9d8220b 100644
--- a/packages/core/src/config.test.ts
+++ b/packages/core/src/config.test.ts
@@ -306,6 +306,32 @@ test("validateConfig rejects unknown keys in a custom rule", () => {
   ).toThrow();
 });
 
+test("validateConfig accepts localization checks and rejects empty locales", () => {
+  const base = {
+    site: { url: "http://localhost:3000" },
+    framework: { type: "next", router: "app" },
+    cms: { provider: "prismic", repositoryName: "demo" },
+    routes: [{ type: "page", pattern: "/:uid", getPath: () => "/about" }],
+  };
+
+  const config = validateConfig({
+    ...base,
+    checks: {
+      localization: {
+        locales: ["en", "fr"],
+        groupField: "translationKey",
+        types: ["page"],
+        severity: "warning",
+      },
+    },
+  });
+  expect(config.checks?.localization?.locales).toEqual(["en", "fr"]);
+
+  expect(() =>
+    validateConfig({ ...base, checks: { localization: { locales: [] } } }),
+  ).toThrow();
+});
+
 test("validateConfig rejects unsupported SEO check sub-options", () => {
   expect(() =>
     validateConfig({
diff --git a/packages/core/src/config.ts b/packages/core/src/config.ts
index d57222e..4477abb 100644
--- a/packages/core/src/config.ts
+++ b/packages/core/src/config.ts
@@ -313,6 +313,16 @@ const checksSchema = z
       .optional(),
     relationships: z.array(relationshipRuleSchema).optional(),
     custom: z.array(customRuleSchema).optional(),
+    localization: z
+      .object({
+        locales: z.array(z.string().min(1)).min(1),
+        localeField: z.string().min(1).optional(),
+        groupField: z.string().min(1).optional(),
+        types: z.array(z.string().min(1)).min(1).optional(),
+        severity: z.enum(["error", "warning", "info"]).optional(),
+      })
+      .strict()
+      .optional(),
   })
   .strict()
   .optional();
diff --git a/packages/core/src/diagnostics.ts b/packages/core/src/diagnostics.ts
index 52b3af2..81285fe 100644
--- a/packages/core/src/diagnostics.ts
+++ b/packages/core/src/diagnostics.ts
@@ -193,6 +193,14 @@ const explanations: DiagnosticExplanation[] = [
       "A relationship rule expected a document to have one or more matching related records, but cms-lab found fewer than the configured minimum.",
     fix: "Check the related CMS collection, the join fields in checks.relationships, and whether the related content should be published or active.",
   },
+  {
+    code: "CMS-LOCALE-MISSING",
+    severity: "warning",
+    title: "Content is missing a locale translation",
+    meaning:
+      "Localization validation is enabled (checks.localization) and a content group does not have a published document in every configured locale. Only published documents count, so a translation that exists only as a draft is reported as missing.",
+    fix: "Publish the missing locale translation, or narrow checks.localization (locales, types, or groupField) to match how the project links translations.",
+  },
   {
     code: "CMS-RELATIONSHIP-UNPUBLISHED",
     severity: "warning",
diff --git a/packages/core/src/scan.test.ts b/packages/core/src/scan.test.ts
index b37c84d..e6c4f85 100644
--- a/packages/core/src/scan.test.ts
+++ b/packages/core/src/scan.test.ts
@@ -2233,3 +2233,112 @@ test("structured-data validation reports routes with no JSON-LD as info", async
   const diag = result.diagnostics.find((d) => d.code === "SEO-JSONLD-MISSING");
   expect(diag?.severity).toBe("info");
 });
+
+async function scanForLocalization(
+  localization: unknown,
+  docs: Array<{
+    id: string;
+    uid?: string;
+    status: "published" | "draft";
+    data: Record<string, unknown>;
+  }>,
+) {
+  return scanDocuments({
+    config: {
+      ...baseConfig,
+      checks: {
+        routes: false,
+        seo: false,
+        a11y: false,
+        images: false,
+        fields: false,
+        localization,
+      },
+    } as never,
+    project: {
+      framework: "next",
+      router: "app",
+      rootDir: "/site",
+      appDir: "/site/app",
+    },
+    documents: docs.map((d) => ({ type: "page" as const, ...d })),
+    fetch: async () => new Response("ok"),
+  });
+}
+
+test("localization flags a content group missing a published locale", async () => {
+  const result = await scanForLocalization({ locales: ["en", "fr", "de"] }, [
+    { id: "en", uid: "about", status: "published", data: { locale: "en" } },
+    { id: "fr", uid: "about", status: "published", data: { locale: "fr" } },
+    // de exists only as a draft -> not live -> missing.
+    { id: "de", uid: "about", status: "draft", data: { locale: "de" } },
+  ]);
+
+  const diag = result.diagnostics.find((d) => d.code === "CMS-LOCALE-MISSING");
+  expect(diag?.severity).toBe("warning");
+  expect(diag?.message).toContain("de");
+  expect(diag?.message).toContain("page/about");
+});
+
+test("localization is silent when every locale is published", async () => {
+  const result = await scanForLocalization(
+    { locales: ["en", "fr"], groupField: "translationKey" },
+    [
+      {
+        id: "en",
+        uid: "about-en",
+        status: "published",
+        data: { locale: "en", translationKey: "about" },
+      },
+      {
+        id: "fr",
+        uid: "a-propos",
+        status: "published",
+        data: { locale: "fr", translationKey: "about" },
+      },
+    ],
+  );
+
+  expect(
+    result.diagnostics.filter((d) => d.code === "CMS-LOCALE-MISSING"),
+  ).toEqual([]);
+});
+
+test("localization skips documents without a locale and respects --skip", async () => {
+  const docs = [
+    {
+      id: "en",
+      uid: "about",
+      status: "published" as const,
+      data: { locale: "en" },
+    },
+    // No locale field -> ignored entirely.
+    { id: "x", uid: "settings", status: "published" as const, data: {} },
+  ];
+
+  const flagged = await scanForLocalization({ locales: ["en", "fr"] }, docs);
+  expect(
+    flagged.diagnostics
+      .filter((d) => d.code === "CMS-LOCALE-MISSING")
+      .map((d) => d.path),
+  ).toEqual(["localization.page.about"]);
+
+  const skipped = await scanDocuments({
+    config: {
+      ...baseConfig,
+      checks: { routes: false, localization: { locales: ["en", "fr"] } },
+    } as never,
+    project: {
+      framework: "next",
+      router: "app",
+      rootDir: "/site",
+      appDir: "/site/app",
+    },
+    documents: docs.map((d) => ({ type: "page" as const, ...d })),
+    filters: { skip: ["localization"] },
+    fetch: async () => new Response("ok"),
+  });
+  expect(
+    skipped.diagnostics.filter((d) => d.code === "CMS-LOCALE-MISSING"),
+  ).toEqual([]);
+});
diff --git a/packages/core/src/scan.ts b/packages/core/src/scan.ts
index a19f5ba..6a893cd 100644
--- a/packages/core/src/scan.ts
+++ b/packages/core/src/scan.ts
@@ -115,6 +115,10 @@ export async function scanDocuments(
     );
   }
 
+  if (shouldRunCheck("localization", options.config, options.filters)) {
+    diagnostics.push(...checkLocalization(options.config, documents));
+  }
+
   return {
     project: options.project,
     documents,
@@ -1147,6 +1151,99 @@ function relationshipRules(config: CmsLabConfig): RelationshipRule[] {
   return config.checks?.relationships ?? [];
 }
 
+/**
+ * Locale-parity check. Groups published documents by a translation key
+ * (`groupField`, defaulting to `document.uid`) and flags any group that does
+ * not have a live document in every configured locale. Only published
+ * documents count toward a locale being present, so a translation that exists
+ * only as a draft is reported as missing.
+ */
+function checkLocalization(
+  config: CmsLabConfig,
+  documents: CMSDocument[],
+): Diagnostic[] {
+  const options = config.checks?.localization;
+  if (!options) {
+    return [];
+  }
+
+  const localeField = options.localeField ?? "locale";
+  const expected = [...new Set(options.locales)];
+  const typeFilter = options.types ? new Set(options.types) : undefined;
+
+  const groups = new Map<
+    string,
+    {
+      type: string;
+      groupKey: string;
+      locales: Set<string>;
+      sample: CMSDocument;
+    }
+  >();
+
+  for (const document of documents) {
+    if (document.status !== "published") {
+      continue;
+    }
+    if (typeFilter && !typeFilter.has(document.type)) {
+      continue;
+    }
+
+    const data = asRecord(document.data) ?? {};
+    const localeValue = readCmsDataPath(data, localeField);
+    if (typeof localeValue !== "string" || localeValue.trim().length === 0) {
+      continue;
+    }
+    const locale = localeValue.trim();
+
+    const groupValue = options.groupField
+      ? readCmsDataPath(data, options.groupField)
+      : document.uid;
+    const groupKey =
+      groupValue === undefined || groupValue === null
+        ? undefined
+        : String(groupValue);
+    if (!groupKey) {
+      continue;
+    }
+
+    const key = `${document.type}:${groupKey}`;
+    const existing = groups.get(key);
+    if (existing) {
+      existing.locales.add(locale);
+    } else {
+      groups.set(key, {
+        type: document.type,
+        groupKey,
+        locales: new Set([locale]),
+        sample: document,
+      });
+    }
+  }
+
+  const diagnostics: Diagnostic[] = [];
+
+  for (const group of groups.values()) {
+    const missing = expected.filter((locale) => !group.locales.has(locale));
+    if (missing.length === 0) {
+      continue;
+    }
+
+    const present = [...group.locales].sort().join(", ") || "none";
+    diagnostics.push(
+      createDiagnostic({
+        severity: options.severity ?? "warning",
+        code: "CMS-LOCALE-MISSING",
+        message: `Content group ${group.type}/${group.groupKey} is missing locale(s): ${missing.join(", ")} (published: ${present})`,
+        path: `localization.${group.type}.${group.groupKey}`,
+        source: sourceFor(config, group.sample),
+      }),
+    );
+  }
+
+  return diagnostics;
+}
+
 function summarizeDiagnosticGroups(
   diagnostics: Diagnostic[],
   config: CmsLabConfig,
@@ -1600,7 +1697,11 @@ function shouldRunCheck(
     return isCheckEnabled(config.checks?.images, true);
   }
 
-  if (group === "relationships" || group === "custom") {
+  if (
+    group === "relationships" ||
+    group === "custom" ||
+    group === "localization"
+  ) {
     return true;
   }
 
diff --git a/packages/core/src/types.ts b/packages/core/src/types.ts
index 29c3495..397a2c3 100644
--- a/packages/core/src/types.ts
+++ b/packages/core/src/types.ts
@@ -161,6 +161,25 @@ export type CustomRuleFn = (
 
 export type CustomRule = CustomDeclarativeRule | CustomRuleFn;
 
+/**
+ * Locale-parity check. Groups published documents by a translation key and
+ * flags any group that does not have a live document in every expected locale.
+ */
+export type LocalizationCheckOptions = {
+  /** Locales every content group should have, e.g. ["en", "fr", "de"]. */
+  locales: string[];
+  /** Path in `document.data` to the document's locale. Defaults to "locale". */
+  localeField?: string;
+  /**
+   * Path in `document.data` to the value that links translations of the same
+   * content. Defaults to `document.uid` (common when locales share a slug).
+   */
+  groupField?: string;
+  /** Limit the check to these content types. */
+  types?: string[];
+  severity?: DiagnosticSeverity;
+};
+
 export type PrismicCmsProviderConfig = {
   provider: "prismic";
   repositoryName: string;
@@ -315,6 +334,7 @@ export type CmsLabConfig = {
     fields?: boolean | { required?: RequiredFieldRule[] };
     relationships?: RelationshipRule[];
     custom?: CustomRule[];
+    localization?: LocalizationCheckOptions;
   };
 };
 
@@ -352,7 +372,8 @@ export type CheckGroup =
   | "images"
   | "fields"
   | "relationships"
-  | "custom";
+  | "custom"
+  | "localization";
 
 export type ScanFilters = {
   types?: string[];
diff --git a/packages/reporter/src/index.ts b/packages/reporter/src/index.ts
index 2f02e8d..d63d3ec 100644
--- a/packages/reporter/src/index.ts
+++ b/packages/reporter/src/index.ts
@@ -644,6 +644,7 @@ function groupDiagnostics(diagnostics: Diagnostic[]): DiagnosticGroup[] {
     "routes",
     "fields",
     "relationships",
+    "localization",
     "seo",
     "a11y",
     "custom",
@@ -677,6 +678,10 @@ function groupForDiagnostic(diagnostic: Diagnostic): string {
     return "relationships";
   }
 
+  if (diagnostic.code.startsWith("CMS-LOCALE")) {
+    return "localization";
+  }
+
   if (diagnostic.code.startsWith("SEO-")) {
     return "seo";
   }