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/config/feature-flags.config.ts

@@ -34,6 +34,12 @@ const FeatureFlagsSchema = z.object({
     required_error:
       'Provide the variable NEXT_PUBLIC_ENABLE_TEAM_ACCOUNTS_BILLING',
   }),
+  languagePriority: z
+    .enum(['user', 'application'], {
+      required_error: 'Provide the variable NEXT_PUBLIC_LANGUAGE_PRIORITY',
+      description: `If set to user, use the user's preferred language. If set to application, use the application's default language.`,
+    })
+    .default('application'),
 });
 
 const featuresFlagConfig = FeatureFlagsSchema.parse({
@@ -65,6 +71,9 @@ const featuresFlagConfig = FeatureFlagsSchema.parse({
     process.env.NEXT_PUBLIC_ENABLE_TEAM_ACCOUNTS_BILLING,
     false,
   ),
+  languagePriority: process.env.NEXT_PUBLIC_LANGUAGE_PRIORITY as
+    | 'user'
+    | 'application',
 } satisfies z.infer<typeof FeatureFlagsSchema>);
 
 export default featuresFlagConfig;