[Asiaplay]

Hi again,

I think the short answer is that you can't without doing some custom programming.
You will find some areas in X-Cart which are not multi-lingual e.g. shipping methods, states (and prior to V4.2.X payment methods weren't multi-lingual either).

Ask X-Cart first perhaps - as I (and doubt few others) have ever added an extra field for users.
But as I say, I think the answer will be some custom coding is needed (in my own case, when required, I got a programmer to make these use language tags - so I put a language tag into the box and update text on the language tags).

Message me if want a programmer contact - cheers, Asiaplay