Hi
I've been using Yedda for about a month or two now and the questions seem to be mostly conducted in English.
I guess you can really use any language you like but it's plain that Yedda is now a worldwide site (more so to come with the AOL takeover) so asking in English will probably open your question to a wider range of possible answers.
Hope this helps - it's only based on my relatively limited experience of the site.