implement multi-language support

[flohoch]

add multi-language support (ask Lukas, our first contributor, for details)

[Rasakul]

Lukas here:
The platform should offer multi-language input for Messages.

Delivering to the client could happen in a way that clients sends current language with request

[philprime]

As a message consists of multiple fields, e.g. title and message, we have two options for localizing a message:

1. Adding a localization requires to set all fields again, basically each message has a relation to n child messages, which are edited individually.
2. Each field can be localized individually, e.g. the message is set in an organization wide default language, and then a second title for a specific language can be set.