@Hydra is correct, aside from the the determination of the language used. We use the language of the browser installation, which can vary from the OS language setting. We don't process IP addresses, draw.io is entirely client-side.
We're going to add two things.
A language menu option and:
Peristence of any specific language selection. That means if you use the ?lang= flag or language menu we will persist that using local storage and use that setting for that browser until the user manually overrides it by setting a different value via either of the two methods.
That will be deployed within the next week.
Edit: There's now a language menu option. Just select the language you want and that's stored in a cookie. On next refresh the menu will show in that language.