Add custom X-LANGUAGE property

Is your feature request related to a problem? Please describe.

Yes, at the moment, there is no way to set a language for a event or appointment booking. So it is not easy to determine the appropriate language for event messages. In instance where someone is working with international clients, they might prefer to have their NC instance in German, but send out invitation to clients in a different language like English, or any other language they speak.

Describe the solution you'd like

The vCalendar RFC does have a LANGUAGE parameter that can be added to the SUMMARY or LOCATION property, but this parameter specifies the language for that specific instance of the property and not the entire event. But the RFC can be extended with a custom property "X-LANGUAGE".

The solution would be to add the following,

• Add support in the backend to read the "X-LANGUAGE"
• Add the ability to set the language in the calendar editor
• Add the ability for a person to choose the language in the appointment selection screen (maybe based on the nc users spoken languages or all available nc languages)

Additionally this functionality could also be implemented on a per organizer and attendee basis, using the LANGUAGE parameter, allowing each person to receive messages in their native language.

The issue identifies backend support, the calendar editor, and the appointment selection screen as affected entry points. Start by tracing how event languages and appointment messages currently flow through those areas; done means X-LANGUAGE can be read, configured, and selected as described, with the per-organizer and attendee behavior clarified.