sorry for the confusion

but that makes no sense, user language should be system-wide and not session specific.

I don't know any website which makes user-language session specific, because that makes no sense

Pooh ... 11 posts to find that actually you're logged in to both devices simultaneously ... ?!? ;)

(you wrote: and then login to another device,)

Thus, it's okay. Each device is keeping its own view of the member data right after being logged-in in a separate session as long as that session is alive.

No

I am logged in to device one and device two at the same time, then I change the language on device one to English and refresh the page on device 2 and still have the old language German

Trying to clarify...
There are 2 devices, D1 and D2.

The last time you've been logged in to D1 you had firstname=Dominik, language=German.
Then you logged off from D1.

You log in to D2
You change your firstname to Kinimod and the language to French and save these settings on the profile page.

You log in to D1
And you still get firstname=Dominik, language=German ?

and it's not making really sense, because when I change my language on facebook (for example) it is changed in my account, not on my device -> it should be stored in database

But in the past it worked? Did you changed that?

that is fine because other device using different session.

When I logout and login again on second device, language is set correctly

When you change langauge on one device, you are loggedin into other device same time? what happen when you logout and login again into other device?

I updated to newest version 7.6 but still not working