Add information around translations

Credits of those texts go to:
* https://code.briarproject.org/briar/briar/-/blob/4e5f2e31df85cb6d56559bb99bc921d322f6a1af/TRANSLATION.md
* !65 (comment 41992)

Related to #65
3 jobs for translation-stuff in 3 minutes and 34 seconds (queued for 2 seconds)