Quote Originally Posted by todoonada View Post
Any error in input of the cc data is handled by https://js.squareup.com/v2/paymentform which is linked to from jscript_square.php.
How can I change the language of the error messages? Error messages are
  • Credit card number is not valid
  • CVV is not valid
  • Expiration date is not valid

I need Japanese.
I asked Square about it, and they said:
Quote Originally Posted by SquareSupport
Currently all payment form errors are returned in English; we don’t offer any translations at this time, so it would be up to you to provide those translations if needed
So I guess that means needing to write some custom javascript, which you'd fire from jscript_square.php before the paymentform js is loaded.