We use English, and I have integrated with 3rd party shipping software (UPS Worldship, Fedex Ship Manager, Endicia, etc) that only support Western charsets. So if a customer enters their address in Cyrillic or Chinese, nopCommerce displays it fine, however upon importing the address into the 3rd party software via SQL it displays as gibberish or question marks.
For example, I have an order needing to ship to Japan:
横浜市鶴見区鶴見中央
Which the 3rd party shipping software sees as ??????????, so the shipping clerk has to open up google translate and copy/paste.
I can't think of any way to force a customer to use a Western charset during checkout, and it's probably not a good idea even if it's possible.
Something like google cloud translation API might be a possibility, but would involve detecting foreign charsets during checkout and translating them before they are stored to the database, which is more overhead to the checkout process, not to mention it's probably error prone.
Perhaps just a note in the shipping address view to "please enter your address in English", but of course that is sure to either go unnoticed or possibly offend.