System language resetting to SIM language after reboot

I recently got a new phone, a Landvo XM100 Plus. It sometimes reboots randomly which doesn't bother me much, but what's annoying is that it keeps setting the system language to German. I did not have these issues on my old phone (Samsung Galaxy S3), and I don't have the problem when there is no SIM Card inserted. However, I'm using the same SIM that I used in my old phone, so I'm really confused as to what is going on. I tried googling the issue and I ended up at a few different boards with everyone basically scratching their heads. Could it be the custom ROM the manufacturer uses? I don't quite want to flash my phone with a different OS right now since I still got a warranty. Any ideas?