I have solved this issue that was caused by Newsletter plugin. There you can activate a template for all system messages. But if you do the mails are empty.
Now I get only English text.
I have installed wpuf-nl_NL.po wpuf-nl_NL.mo in wp-content\plugins\wp-user-frontend-pro\languages\
but it not became active.
Any idea how to get Dutch text into registration and other WPUF PRO mails?