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 <[email protected]>
Co-authored-by: Marco A. <[email protected]>

internal/api/http/middleware/instance_interceptor_test.go

@@ -338,6 +338,10 @@ func (m *mockInstance) DefaultLanguage() language.Tag {
 	return language.English
 }
 
+func (m *mockInstance) AllowedLanguages() []language.Tag {
+	return []language.Tag{language.English}
+}
+
 func (m *mockInstance) DefaultOrganisationID() string {
 	return "orgID"
 }