>And no, we don't use "postal numbers for humans".

An email, a phone number, a tax or social security number, demographic identifier, billing/contract number or combination of them.

All of those will help you tell Stefan from Štefan in the most practical situations.

>But if someone runs that script above and transliterates "š" to "s" it can cause confusion.

It's not nice, it will certainly make Štefan unhappy, but it's not like you will debit the money from the wrong account or deliver to a different address or contact the wrong customer because of that.