Every field has an option to personalize the field and the error message except for the form itself.
I argue that these two lines:
There was a problem with your submission.
Errors have been highlighted below
Should be on the advanced part of the form for customization.
This would allow everybody to internationalize the forms and fields on the fly just by changing the defaults.
One could also duplicate the form, change the defaults and voila another language. (This would also work well with qtranslate etc.)
Right now I'm not seeing other option but to "hack" the plugin files! :|
(if we go deeper, I would ask if the address could also work like this but I guess it could be more tricky).