How to fix Phones language resetting to SIM language after reboot?
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. 



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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