feat: Ability to set installation-wide default locale (#1433)

* Dashboard locale can be set via env variable
* Change account locale based on registration page
* Set account locale if available

Co-authored-by: Pranav Raj Sreepuram <pranavrajs@gmail.com>

app/controllers/concerns/switch_locale.rb

@@ -0,0 +1,38 @@
+module SwitchLocale
+  extend ActiveSupport::Concern
+  included do
+    around_action :switch_locale
+  end
+
+  private
+
+  def switch_locale(&action)
+    # priority is for locale set in query string (mostly for widget/from js sdk)
+    locale ||= locale_from_params
+    # if locale is not set in param, lets try account
+    locale ||= locale_from_account(@current_account)
+    # if locale is not set in account, let's use DEFAULT_LOCALE env variable
+    locale ||= locale_from_env_variable
+    # if nothing works we rely on default locale
+    locale ||= I18n.default_locale
+    # ensure locale won't bleed into other requests
+    # https://guides.rubyonrails.org/i18n.html#managing-the-locale-across-requests
+    I18n.with_locale(locale, &action)
+  end
+
+  def locale_from_params
+    I18n.available_locales.map(&:to_s).include?(params[:locale]) ? params[:locale] : nil
+  end
+
+  def locale_from_account(account)
+    return unless account
+
+    I18n.available_locales.map(&:to_s).include?(account.locale) ? account.locale : nil
+  end
+
+  def locale_from_env_variable
+    return unless ENV.fetch('DEFAULT_LOCALE', nil)
+
+    I18n.available_locales.map(&:to_s).include?(ENV.fetch('DEFAULT_LOCALE')) ? ENV.fetch('DEFAULT_LOCALE') : nil
+  end
+end