feat(api): return allowed_languages (#11553)

# Which Problems Are Solved

While Zitadel provides a possibility to restrict certain languages to be
used, the corresponding list could not be retrieved in the settings
service (v2). This blocked login v2 implementations from respecting the
list and they would always use all available languages.

# How the Problems Are Solved

- Retrieve the list when checking the instance and pass it into the
context.
- Return it as part of the existing `GetGeneralSettingsResponse`
- This allows us to remove an additional query in some other cases /
endpoints.

# Additional Changes

none

# Additional Context

- required for https://github.com/zitadel/zitadel/pull/11372
- backport to v4.x

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Marco A. <marco@zitadel.com>

proto/zitadel/settings/v2/settings_service.proto

@@ -701,6 +701,8 @@ message GetGeneralSettingsResponse {
   ];
 
   // The list of supported languages.
+  // Note that the instance might restrict the languages further
+  // only allowing a subset of these languages to be used.
   // The format is a BCP 47 language tag (e.g. "en", "de", "fr-CH").
   repeated string supported_languages = 3 [
     (grpc.gateway.protoc_gen_openapiv2.options.openapiv2_field) = {
@@ -711,6 +713,16 @@ message GetGeneralSettingsResponse {
   // The unique identifier of the default organization.
   // The default organization is used to assign new users to an organization if no other organization is specified.
   string default_organization_id = 4;
+
+  // The list of allowed languages for the instance.
+  // This is a subset of the supported languages to be used in the instance
+  // e.g. for user selection during registration or language detection in the UI.
+  // The format is a BCP 47 language tag (e.g. "en", "de", "fr-CH").
+  repeated string allowed_languages = 5 [
+    (grpc.gateway.protoc_gen_openapiv2.options.openapiv2_field) = {
+      example: "[\"en\", \"de\", \"it\"]"
+    }
+  ];
 }
 
 message GetSecuritySettingsRequest{}