Croatian LARP has been quite international for the last year. We had very few major LARPs without at least one visitor from another country.
Of course, there was never any problem with our visitors from Serbia, as Serbian is almost the same language (linguistically, if not politically).
It was Martin who was our first regular player from another country. He doesn't speak Croatian well, but he understands some as he also speaks Czech and Slovenian. With him around, the players fell into a complex, yet very interesting and rewarding pattern - despite the fact that all our players speak English, some chose not to do so in-character. In-game communication was still possible between them to a degree, and players basically built those communication diffilculties in-character on purpose to enrich character interaction in a non-standard way and make our story more interesting.
However, that approach didn't work once we got visited by Sarah last October, who didn't speak Croatian or any related language. It required a different approach, and not everyone was able to react accordingly just yet. Me and some other players did plenty of translation on that event to make sure she understood what was going on. After it, we basically decided to relax our sometimes strict roleplaying requirements, as enjoyment of our visitors should come first - so if you don't speak Croatian and can't understand what's going on, our players will gladly help you. And in a quicker manner than when Sarah was here.
With the high likelyhood that we're gonna get visited by Slovenians this year, and possibly even Bulgarians, things are certainly getting interesting, as they are closely related to Croatian, allowing basic communication, but sometimes details can get lost. Had a similar experience this weekend in Slovenia - plenty of Slovenian was understandable to me, but not everything, so sometimes we've fallen back on English (and I did get that feeling they understood us better than vice versa). Older generations of Croats would probably find this less of an issue than we did since they got much more exposure to Slovenian (and vice versa). But we can still get understood, there's English to fall back on, and their presence would enrich our LARPs - and Bulgarian players would be in a similar situation here.
There are plenty of ways to deal with linguistic diversity on LARP. Yet most of them are fun, interesting and enrich our game as much as our visitors themselves.