The language barrier can be bigger than you think. An example being that a lot of EU mixes tend to be people from the same nationality because they don't want to comm English. And for many of them that's their only practise in comp tf2 outside of officials/scrims (which is also often on teams that do not comm English. Many stick together even if it means not improving as fast or being limited to a small player pool to pick from)
Only reason I'm on a team this season is because I got introduced to the Dutch mix circlejerk. Not that I mind the comm language, but there was a relatively large demand for Dutch mixes which I was unaware of before. They all can comm English very well, but avoid mixing with non-dutch speaking players unless there's no other way. Very often we mix against Finnish only, Russian only, etc mixes because they'd just rather comm in their own language
funhaver1998most people speak enough to call
This is based on your experience talking to those who can, but in reality a decent chunk of players does not. You just don't notice it because they're actively avoiding having to talk English so you never get in contact with them