Maybe this will be useful to people here as I was fighting with this issue for the past several days (in my case the main language was Polish and was SUPPOSED to be Polish but was mixed with English for frankly insane reason, as you will see):
Okay, solved the issue. You want to know what the problem was? Hold on to your seat, and if you are standing, please sit down.
Are you ready?
The cause of the mixed language was because I’ve set NUMBER FORMAT IN THE REGIONAL SETTINGS (yes, you’ve read this correctly) to something else than Polish, because I prefer thousands to be separated by spaces.
Could this issue be any less obvious?