With the initial language 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.
There are menus to change the default language for your browser in two places. At the bottom right of the initial splash screen there is a little "world" icon:
You can select a new language and refresh the page for that to take effect.
The world icon also appears at the right of the menu bar while using the application:
Again, you need to refresh the page for the language change to take effect. Note that you also need cookies to be enabled for the setting to persist.