At the bottom of the window (on Mac anyway) is a filter. Type the file ending (azw or mobi) to just see those files, or -epub to NOT see epubs.
As for language, that'll either be part of the file name or not. I'd assume based on how I name files that they are EN if the file name appears EN, and if I was using the English name for a German language file, it would have DE (caps) somewhere in the file name. But that's not a Soulseek issue, to be honest.
Or am I missing something in your question?