fix(core): pass locale to beforeRequest for Accept-Language header

Inside unstable_cache, getLocale() fails silently (returns undefined)
because it calls headers() internally. Pass the explicit locale from
client.fetch() through to the beforeRequest callback so Accept-Language
is set correctly even inside cached contexts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jorgemoya

core/client/index.ts

@@ -47,7 +47,7 @@ export const client = createClient({
     // We use the default channelId as a fallback, but it is not ideal in some scenarios.
     return getChannelIdFromLocale(resolvedLocale) ?? defaultChannelId;
   },
-  beforeRequest: async (fetchOptions) => {
+  beforeRequest: async (fetchOptions, locale) => {
     // We can't serialize a `Headers` object within this method so we have to opt into using a plain object
     const requestHeaders: Record<string, string> = {};
 
@@ -67,10 +67,12 @@ export const client = createClient({
       }
     }
 
-    const locale = await getLocale();
+    // Use explicit locale when available (required inside unstable_cache where
+    // getLocale() would fail), fall back to next-intl for normal requests.
+    const resolvedLocale = locale ?? (await getLocale());
 
-    if (locale) {
-      requestHeaders['Accept-Language'] = locale;
+    if (resolvedLocale) {
+      requestHeaders['Accept-Language'] = resolvedLocale;
     }
 
     return {

packages/client/src/client.ts

@@ -23,6 +23,7 @@ interface Config<FetcherRequestInit extends RequestInit = RequestInit> {
   getChannelId?: (defaultChannelId: string, locale?: string) => Promise<string> | string;
   beforeRequest?: (
     fetchOptions?: FetcherRequestInit,
+    locale?: string,
   ) => Promise<Partial<FetcherRequestInit> | undefined> | Partial<FetcherRequestInit> | undefined;
   onError?: (
     error: BigCommerceGQLError,
@@ -52,6 +53,7 @@ class Client<FetcherRequestInit extends RequestInit = RequestInit> {
   private getChannelId: (defaultChannelId: string, locale?: string) => Promise<string> | string;
   private beforeRequest?: (
     fetchOptions?: FetcherRequestInit,
+    locale?: string,
   ) => Promise<Partial<FetcherRequestInit> | undefined> | Partial<FetcherRequestInit> | undefined;
   private onError?: (
     error: BigCommerceGQLError,
@@ -133,7 +135,7 @@ class Client<FetcherRequestInit extends RequestInit = RequestInit> {
       locale,
     );
     const { headers: additionalFetchHeaders = {}, ...additionalFetchOptions } =
-      (await this.beforeRequest?.(fetchOptions)) ?? {};
+      (await this.beforeRequest?.(fetchOptions, locale)) ?? {};
 
     const response = await fetch(graphqlUrl, {
       method: 'POST',