Did the books ever address the language issue? After all, people didn't speak English at many places that Jack and Annie went to but they're able to communicate with everyone just fine. Is it part of the magic (like the way their clothes change)?
That's probably the case. Either that, or it's meant to be an Acceptable Break from Reality, of which there are a lot (I mean, language aside, two white American kids strolling around with no adult supervision is definitely going to raise eyebrows in a lot of places/time periods).