I can add my anecdote to the language barrier points.
At work we speak English, everybody speak English all the time, all docs are in English, all meetings are in English. There's an occasional German email every now and then but people will switch.
When we go to a "team building" retreat, all the same teammates that happily chatted with us expats in English just switch to German 100%, full stop. You can come and stand at the edge of a group chatting in German and they will look at you and continue, knowing you stand there not understanding, without batting an eye.
At work we speak English, everybody speak English all the time, all docs are in English, all meetings are in English. There's an occasional German email every now and then but people will switch.
When we go to a "team building" retreat, all the same teammates that happily chatted with us expats in English just switch to German 100%, full stop. You can come and stand at the edge of a group chatting in German and they will look at you and continue, knowing you stand there not understanding, without batting an eye.