There is a language setting in the user profile field, which is the default language for the user
There is a default language setting in admin (lang) which is the default language for the site (= when user is not logged in)
On top of that, you can enable browser detection of your language (autolang). In that case, the language is what your browser thinks your language is.
And then you can set a specific language for every course if you want.
So lots of places where it can go wrong
My suggestion is:
- Look at your profile when logged in and make sure that is what you want.
- Perform a search for" language" (or whatever the word is in the language you are looking at) in side administration, looking through all the settings and try to understand them. If you don't - leave them as default.
- Disable in those settings the browser detection setting (autolang)
So I hope there is something in this too long post that gives you a clue to solve your problem