It's always interesting to watch how a bunch of non-native speakers of English from different countries sitting in a room can talk to each easily, but when a Brit or an American joins, the conversation immediately collapses.

In what way? What I've seen is the native English speaker bends over backwards to explain each and every idiom or try to not use them in the first place.

As a non native English speaker: British accent is harder to understand (I know there are many accents in the UK). American accent is easier to understand. Idioms are equally harder to understand in both.

For example “bend over backwards”. I get the meaning, but my brain would never produce that phrase. I would say something like “adapts”, “compromises”, etc.