I’m fairly confident it has to do with the OS and the way MindtheGapps works. Something from the GBoard/Swiftkey keyboards is not giving information in real time to the system about the current language when using a multilingual keyboard.
Like in this example:
The keyboard allows you to use multiple languages at the same time and recognises the language as you type the first word or two and then selects the language for the autocorrect. This is a great feature but unfortunately, I haven’t seen it working. Choosing a single language keyboard with the default AOSP board does work and changes language as expected but… The convenience of having the 3 languages I use for work be always active is now lost 
Yes, that’s the one. According to this post it seems as though the iodé team has found some sort of fix. I don’t know whether the fix is still working…
Unfortunately, I’d like to be able to use google pay, that’s also why I’m not really willing to sort out the other problems. Especially since every guide I’ve found over the subject from 2017 to today says something along the lines of “Use this at your own risk, this workaround may or may not stay working with future updates as google may change the safetynet and other google pay API requirements.” Given the multitude of different methods I managed to find over the subject. I decided not to bother trying.