Thread:Project:Support desk/Is there a Template:USERLANG function similar to en?/reply

Not easily.

There is a way though, doing will translcude mediawiki:lang/. So if you create mediawiki:lang/en, mediawiki:lang/fr, etc with the relavent codes, would work like userlanguage (Yes this is a horrible ugly hack. Its what is done on Wikimedia commons).