Considering the wide user base of X-cart, I think German, French, Spanish and Russian subforums would be good choices. That would hit a wide array of countries.
Actually if you think about it, more people speak Mandarin and Spanish than English! I am glad the main language is English though, I am a little partial to it!
http://en.wikipedia.org/wiki/List_of_languages_by_number_of_native_speakers