[FEATURE] render sessions language, if there are sessions of different languages planned in the schedule

[MikkCZ]

As a conference organizer, I would like to be able to render the session language in the schedule, so the event visitors know some sessions are held in different language.

It would probably be a waste of space, if all sessions have the language indicated (either as text or by a flag), so this should be configurable. Or the schedule could just render the indication for sessions which language differ from the locale the schedule itself is using.

[rixx]

Adding languages is a pretty big undertaking:

• We'd need to also add a language filter (and thus, filtering in general), as adding this information without making it actionable is not good.
• Filters in general mean a filter menu, clear indication that the schedule is filtered, filter persistance in URLs, filter removal etc.
• Languages cannot be rendered as flags. Thus they add visual clutter and we need to carefully find a place for them.
• Languages could be rendered as language code, except that many people won't understand language codes, especially longer ones. Option 1: Language code, and choose shortest unique code (e.g. "ZH" for "zh_Hant", unless "zh_Hans" is also in use). Option 2: Language names, requiring localisation into all pretalx-supported languages.