I think this is related to https://boldminded.com/support/ticket/716

Force CP to default language is off.

Say I’m copying some content from one entry (X) to another (Y).  I have 2 languages spanish and english.  Spanish is the default.  I open entry X to the edit screen and spanish is showing, then I open entry Y and have to switch to english to get the content I want.  At this point entry X is showing spanish, and entry Y is showing english.  (But, I suspect I have just set the CP cookie to english for my current CP language)  I finish copying content and hit publish on entry X expecting it to save into spanish (because that is what is showing in the dropdown), but I end up with the spanish content rows from the matrix fields appended to the matrix fields in the english translation of entry X rather then saving into spanish.

Basically if you switch the CP language in one window, then come back to an entry open in another window and expect it to save in the language shown in the dropdown it doesn’t, because you’ve switched the language and the other window didn’t know about it.