Add language priority feature flag

This commit adds a new 'languagePriority' key to the feature flags configuration. It determines whether to use user's preferred language or the application's default language. Adjustments to the language selection process according to new flag have been made in the 'createInstance' function in 'i18n.server.ts'. Additionally, updates have been made to 'README.md' and '.env' files to reflect these changes.

apps/web/lib/i18n/i18n.server.ts

@@ -7,6 +7,7 @@ import {
   parseAcceptLanguageHeader,
 } from '@kit/i18n/server';
 
+import featuresFlagConfig from '~/config/feature-flags.config';
 import {
   I18N_COOKIE_NAME,
   getI18nSettings,
@@ -15,6 +16,12 @@ import {
 
 import { i18nResolver } from './i18n.resolver';
 
+/**
+ * @name priority
+ * @description The language priority setting from the feature flag configuration.
+ */
+const priority = featuresFlagConfig.languagePriority;
+
 /**
  * @name createI18nServerInstance
  * @description Creates an instance of the i18n server.
@@ -24,25 +31,46 @@ import { i18nResolver } from './i18n.resolver';
  * Initialize the i18n instance for every RSC server request (eg. each page/layout)
  */
 function createInstance() {
-  const acceptLanguage = headers().get('accept-language');
   const cookie = cookies().get(I18N_COOKIE_NAME)?.value;
 
-  let language =
-    cookie ??
-    parseAcceptLanguageHeader(acceptLanguage, languages)[0] ??
-    languages[0];
+  let selectedLanguage: string | undefined = undefined;
+
+  // if the cookie is set, use the language from the cookie
+  if (cookie) {
+    selectedLanguage = getLanguageOrFallback(cookie);
+  }
+
+  // if not, check if the language priority is set to user and
+  // use the user's preferred language
+  if (!selectedLanguage && priority === 'user') {
+    const userPreferredLanguage = getPreferredLanguageFromBrowser();
+
+    selectedLanguage = getLanguageOrFallback(userPreferredLanguage);
+  }
+
+  const settings = getI18nSettings(selectedLanguage);
+
+  return initializeServerI18n(settings, i18nResolver);
+}
+
+export const createI18nServerInstance = cache(createInstance);
+
+function getPreferredLanguageFromBrowser() {
+  const acceptLanguage = headers().get('accept-language');
+
+  return parseAcceptLanguageHeader(acceptLanguage, languages)[0];
+}
+
+function getLanguageOrFallback(language: string | undefined) {
+  let selectedLanguage = language;
 
   if (!languages.includes(language ?? '')) {
     console.warn(
       `Language "${language}" is not supported. Falling back to "${languages[0]}"`,
     );
 
-    language = languages[0];
+    selectedLanguage = languages[0];
   }
 
-  const settings = getI18nSettings(language);
-
-  return initializeServerI18n(settings, i18nResolver);
+  return selectedLanguage;
 }
-
-export const createI18nServerInstance = cache(createInstance);