No event default language

[paulmenzel]

[GitHub issue for Angebotsanfrage für Arbeit an Indico: Event default language]

• Currently Indico only has a language setting for users, but no way to indicate the primary language of an event.
  • Add a new event setting with the default language for the event
  • Add a new event setting with the supported languages in the event
  • Add a new user setting to always use the preferred language
  • In end-user-facing parts of the event (i. e. not the management area), these new language settings should behave like this:
    • User setting enabled: Always use the user’s language (ignore supported languages); otherwise use the user’s language if it’s supported, and fall back to the default language if it isn’t.
    • When sending emails to a user, use the same logic to determine the language used there.
    • When sending emails without knowing the user’s language, use the event’s default language.