English language should be lingua franca, comments/questions can be also in other languages, but to make procedure comprehensible to biggest possible audience if using other language ask somebody to translate it to English language.
Common principles used wikiwide (usage of references) applies. E.g. to establish something please quote your sources like this:
- Krešo Golik (May 20 1922 Fužine - September 20 1996 Zagreb) 
- Petar Krelja Golik ISBN 953-6005-21-2 Zagreb 1997.
All sources must not be relevant, today exist samizdat books and yellow papers, so every source can be challenged or marked as not relevant enough.
If all sides in some dispute agree about some fact for which there is no available source/reference, that fact is accepted (e.g. common knowledge).