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Oh well, I managed it myself. Now the comments are filtered by language.
When I choose "english" (xlanguage module), only those comments are displayed which were posted when "english" was chosen.
I added additional variable in /class/commentrenderer.php and assigned a smarty variable in /includes/comment_view.php (didn't know where else).
Comments are still counted in all languages and I have to hack the search and edit more templates, but the main work is done. Pretty close to a real multilingual site now.
Comments can be sorted not only by language using this technique, I guess. However, I'm not sure if the com_exparams column in xoopscomments table is really never used. Will get a surprise someday..
Thank for help, friends