Language
Customers should get an answer they can read. The language on the group is the language the assistant uses with them on the website. It is not the language of the Cusu app. Your account language only changes how you read translations. It does not change what the customer hears.
Cusu suggests a language from the site when you create the group. Change it if that guess is wrong.
Why you usually fix it
A Czech shop should answer in Czech even when a visitor's browser is in English. Turn Detect language automatically off, then pick Assistant language. Do the same for each country group: German for the German shop, Polish for the Polish shop.
Leave detection on only when one group really does serve people in whatever language they brought with them. The widget and the replies then follow the page and the customer. The language list is unused, and it shows From page / customer language.
Both switches are in the group's Basic settings. Only an admin can change them.