Hello piethoed,
For older versions (2.x): This plugin does not support complete translation. In older versions you have to dig in the code and translate the words yourself.
For newer versions (3.x): In the plugin is a translatation file included (tmp-wp-customer-reviews-readme.pot). You can open this file with a translation tool. Notice that the file is actually a .po file (the extension .pot is a mistake I guess).
Hope this helps.
I had the same issue with the WooThemes theme ‘ Superstore’.
The hotfix described above also worked for me (placed in functions.php)
