To the best of my knowledge, if you go to https://developer.mozilla.org you're just served articles in English (that may be because I have English set as my language in my browser, I don't remember what happens if I set it to my actual native language). But I noticed that Google search results actually are pointing me to my native language.
I'm Swedish, and get "redirected" to https://developer.mozilla.org/sv-SE/ if I follow that link, even though I have English set as the language in my browser.
2011-2015 I was on the team at Mozilla responsible for MDN.
At the time it most certainly was not doing any type of IP geolocation; your browser's Accept-Language header was how we initially determined the language to show you.
Edit: And it still does only use Accept-Language. Here's the code: