How to fix Phones 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.

Any ideas?

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 warranty.

Best regards!
I disabled secure start up. 
I guess secure start up uses the default keyboard (instead of SwiftKey that I normally uses) and that the language of the default keyboard depends on the sim card. 



(Click to enlarge)

I am using a cubot power (cheap Chinese crap) with a Thai and a Danish sim card. Before the fix, the language changed to Danish every time I rebooted