Different languages in different sites

As my friend Dave Mehr says, the most foolproof way to ensure that everyone sees the site in one language is to deactivate the other languages of the site.  If you want a less foolproof way to do it just for yourself, but more reliable than changing your account language, you can change your profile language, which is different from the account language.  This used to be done in Delve, but this is the more current procedure. 

This changes the user interface language.  If the site is using the multilingual page publishing feature and it is the language of the content, not the language of the interface, that is in the wrong language, it is because you once used the small content language toggle, and it remembers on that site for a while.  It forgets it when you close all the browser windows and reopen, or if you toggle it back again.