Payment form validation error messages

I'm implementing payment form on my site. I'm running into the following validation issues:

- with radio button field: if the field is required the user has to make a selection. If the user doesn't make a selection, an empty error balloon shows. I have specified a 'Validation Error Message'


- with a field containing the phone number, I have made a validation to check its size (validate[required,custom[integer],minSize[10]])). When the user inputs a shorter number - the error message in the red balloon is in English. I have looked at the translation part, but I could only translate the * Minimum. But I can't find the translation for "characters required" (even not as separate words)


Furthermore, I have seen this more on the forum, it would be very helpful to get some more explanation around custom validators. For this form I need a proper validation for :

- Dutch zip codes (9999XX or 9999 XX)
- European iBan numbers (Bank accounts) - basically something like XX99XXXX9999999999

Where a 9 stands for an integer and X for a character (small or caps)