English in one form or another is pretty much a
lingua franca (which, oddly enough, means "french language") in probably most of the world - but that doesn't mean that you can understand someone else speaking it, or be sure that they understand you speaking it. Loren Wiseman once posted a story about taking a tour in Sweden; English was the common language. There was a Norwegian and a Finn in the tour group, and the Norwegian couldn't understand the Finn's English or vice-versa - but Loren could understand both, and both could understand his American. So, he ended up translating between English and English.
On the flip side, English can be butchered pretty badly and still be understandable. I even wrote an article about doing that, specifically to address in a game-useful way the idea of different languages without going all the way to forcing the players to get involved in translating. See "
Speaking in Tongues: Simulating Dialects in Your Game",
Freelance Traveller, July/August 2017.