Are multiple languages supported?
Yes. You can define as many languages for content as you want on event creation. The first language is the primary language, that is always used if other languages are not filled in.
You can also create mail templates in any language you want. (Hint: just duplicate it and add e.g. -de to the name of the template)
The Event66 Backoffice as well as the contribution self-service forms are currently available on english.