I18n: Add default locale server config option (#51035)

* I18n: Set default locale in server config and expose in grafanaBootData

* put default locale behind feature flag

* update tests now that default locale is behind feature flag

* little bit of PR feedback

* update sample.ini

pkg/setting/setting.go

@@ -397,8 +397,9 @@ type Cfg struct {
 
 	Quota QuotaSettings
 
-	DefaultTheme string
-	HomePage     string
+	DefaultTheme  string
+	DefaultLocale string
+	HomePage      string
 
 	AutoAssignOrg              bool
 	AutoAssignOrgId            int
@@ -1356,6 +1357,7 @@ func readUserSettings(iniFile *ini.File, cfg *Cfg) error {
 	LoginHint = valueAsString(users, "login_hint", "")
 	PasswordHint = valueAsString(users, "password_hint", "")
 	cfg.DefaultTheme = valueAsString(users, "default_theme", "")
+	cfg.DefaultLocale = valueAsString(users, "default_locale", "")
 	cfg.HomePage = valueAsString(users, "home_page", "")
 	ExternalUserMngLinkUrl = valueAsString(users, "external_manage_link_url", "")
 	ExternalUserMngLinkName = valueAsString(users, "external_manage_link_name", "")