Now, the fun starts when you use different browsers :)
The phenomenon described above can be experienced in Opera. The messages displayed are localised but they don't display correct field names.
In Firefox on the other hand the messages are not localised but the fields names are as I have entered in the CMS.
I can't wait to get my laptop near a Windows machine for IE testing :>
This sheds some light on the issue however: in Opera the javascript validation doesn't seem to work - messages show only when I press the "submit" button. On Firefox the javascript validation works.
So, the javascript part is not localised and the server side part of the validation provides incorrect field names.
Where do I start patching the code? :)