At last it works with bbpress-cs_CZ.mo and bbpress-cs_CZ.po in ../wp-content/languages/bbpress folder.
Thank you, this way of export works fine and there are czech strings correctly translated in language files now. I renamed them to bbpress-cs.po and bbpress-cs.mo and have it inserted to folders:
/matia.info/wp-content/languages/bbpress
/matia.info/wp-content/languages/plugins
/matia.info/wp-content/plugins/bbpress/languages
The strings at my page http://www.objasnujeme.cz/registrace/ are unfortunately still only in english “Your username must be unique, and cannot be changed later.” instead of “Vaše uživatelské jméno musí být jedinečné, a nelze ho později měnit.”.
