diff --git a/packages/cli/cli/changes/unreleased/fix-default-locale-manifest-key.yml b/packages/cli/cli/changes/unreleased/fix-default-locale-manifest-key.yml
new file mode 100644
index 000000000000..183bc908d7d4
--- /dev/null
+++ b/packages/cli/cli/changes/unreleased/fix-default-locale-manifest-key.yml
@@ -0,0 +1,5 @@
+# yaml-language-server: $schema=../../../../../fern-changes-yml.schema.json
+
+- summary: |
+    Publish the configured default locale as the docs manifest locale key instead of always using "en".
+  type: fix
diff --git a/packages/cli/generation/remote-generation/remote-workspace-runner/src/__test__/buildLedgerInput.test.ts b/packages/cli/generation/remote-generation/remote-workspace-runner/src/__test__/buildLedgerInput.test.ts
index f552714f21aa..44609f8e9247 100644
--- a/packages/cli/generation/remote-generation/remote-workspace-runner/src/__test__/buildLedgerInput.test.ts
+++ b/packages/cli/generation/remote-generation/remote-workspace-runner/src/__test__/buildLedgerInput.test.ts
@@ -23,15 +23,17 @@ const MINIMAL_ROOT = {
 
 function makeDocsDefinition({
     pages = {},
-    root = MINIMAL_ROOT
+    root = MINIMAL_ROOT,
+    translations
 }: {
     pages?: Record<string, { markdown: string }>;
     root?: unknown;
+    translations?: { defaultLocale: string; translations?: string[] };
 } = {}) {
     // Minimal DocsDefinition shape — only the fields buildLedgerInput reads.
     return {
         pages,
-        config: { root }
+        config: { root, ...(translations != null ? { translations } : {}) }
     } as Parameters<typeof buildLedgerInput>[0]["docsDefinition"];
 }
 
@@ -222,6 +224,52 @@ describe("buildLedgerInput", () => {
         expect(localeEntry.locale).toBe("en");
     });
 
+    it("keys the base segment by the configured default locale", () => {
+        const { localeEntry } = buildLedgerInput({
+            docsDefinition: makeDocsDefinition({
+                translations: { defaultLocale: "en-US", translations: ["en-US", "pt-BR", "ja-JP"] }
+            }),
+            apiDefinitions: new Map()
+        });
+
+        // publishInput.defaultLocale is derived from this value, so the
+        // manifest's defaultLocale and its base locales[] key stay in sync
+        // with the tag readers look up from docs.yml.
+        expect(localeEntry.locale).toBe("en-US");
+    });
+
+    it("preserves a non-English default locale", () => {
+        const { localeEntry } = buildLedgerInput({
+            docsDefinition: makeDocsDefinition({
+                translations: { defaultLocale: "de-DE", translations: ["de-DE", "en"] }
+            }),
+            apiDefinitions: new Map()
+        });
+
+        expect(localeEntry.locale).toBe("de-DE");
+    });
+
+    it("keeps an explicitly passed translation locale, including regional tags", () => {
+        const { localeEntry } = buildLedgerInput({
+            docsDefinition: makeDocsDefinition({
+                translations: { defaultLocale: "en-US", translations: ["en-US", "pt-BR"] }
+            }),
+            apiDefinitions: new Map(),
+            locale: "pt-BR"
+        });
+
+        expect(localeEntry.locale).toBe("pt-BR");
+    });
+
+    it("keys the base segment by en when the site declares no translations", () => {
+        const { localeEntry } = buildLedgerInput({
+            docsDefinition: makeDocsDefinition({ pages: { "page-1": { markdown: "# Hi" } } }),
+            apiDefinitions: new Map()
+        });
+
+        expect(localeEntry.locale).toBe("en");
+    });
+
     it("uses config.root for the root field", () => {
         const { localeEntry } = buildLedgerInput({
             docsDefinition: makeDocsDefinition({ root: MINIMAL_ROOT }),
diff --git a/packages/cli/generation/remote-generation/remote-workspace-runner/src/publishDocsLedger.ts b/packages/cli/generation/remote-generation/remote-workspace-runner/src/publishDocsLedger.ts
index cc645df66b59..588d8880bd82 100644
--- a/packages/cli/generation/remote-generation/remote-workspace-runner/src/publishDocsLedger.ts
+++ b/packages/cli/generation/remote-generation/remote-workspace-runner/src/publishDocsLedger.ts
@@ -123,7 +123,7 @@ export function buildLedgerInput({
     fileManifest,
     fileIdToPath,
     editThisPage,
-    locale = "en"
+    locale
 }: {
     docsDefinition: DocsDefinition;
     git?: DocsPublishGitInput;
@@ -139,7 +139,11 @@ export function buildLedgerInput({
     fileIdToPath?: Map<string, string>;
     /** Raw edit-this-page config from docs.yml, forwarded to LedgerConfig. */
     editThisPage?: { github?: { owner: string; repo: string; branch?: string; host?: string } };
-    /** Locale to stamp on segments. Defaults to "en". */
+    /**
+     * Locale to stamp on segments. When omitted (the base segment), it falls
+     * back to the default locale the docs config declares, and to "en" for
+     * sites that declare no translations.
+     */
     locale?: string;
 }): { localeEntry: LocaleEntry; blobs: Map<string, Buffer> } {
     const blobs = new Map<string, Buffer>();
@@ -225,7 +229,7 @@ export function buildLedgerInput({
         jsFiles: jsFilesRef,
         fileManifest,
         redirects: null,
-        locale,
+        locale: locale ?? docsDefinition.config.translations?.defaultLocale ?? "en",
         git
     };