Django (like other frameworks) listen to specific headers to check for languages (namely :
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Language). That is the same on all devices.
The way we know what language to display the website in is through the browsers sending this information to us. So the browsers have to be setup to include the correct language - which usually is the case. However I have all of my devices setup to display swedish - except firefox on my laptop - which is setup for english. My guess is that the users browser also is setup for english. I would check to see how the same browser on another android phone reacts to your site. You can try this on browserstack for example.