discourse/lib/validators/allow_user_locale_enabled_validator.rb

class AllowUserLocaleEnabledValidator

  def initialize(opts = {})
    @opts = opts
  end

  def valid_value?(val)
    # only validate when enabling setting locale from headers
    return true if val == "f"
    # ensure that allow_user_locale is enabled
    SiteSetting.allow_user_locale
  end

  def error_message
    I18n.t("site_settings.errors.user_locale_not_enabled")
  end

end