/usr/bin/pdfcsplain is linked to xetex but it have to be linked with pdftex (try the original texlive distribution). Link to xetex causes troubles with accented characters.
wow, this bug was there since the beginning of texlive in gentoo (2007...) but was never noticed since there never were multiple possibilities for linking format binaries... I've fixed the eclasses so that now the first one in upstream configuration is always selected, as intented by upstream, which in this case is as you say: link pdfcsplain to pdftex. dev-texlive/texlive-langczechslovak-2015-r1 should fix the issue thanks for the report!
