If you want to trade Pokemon across language barriers, you can use the PokeDex numbers to negotiate. Or at least that's how it's personally been useful to me. The numbers essentially work as a Pokemon's "true name" much in the same way latin names work for animals - if you know the numbering system of another language - which is usually one of the first things you learn - than you can use that to indicate Pokemon despite not knowing foreign names. That works in universe and outside.