We had a report of this feature not working - the language getting reset as soon as you open a message - on Android 7.0.
I checked, and found it to be true (checked on two devices, one with 7.0 the other with 8.1).
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Android apps are not supposed to do this. There is a setting in Andorid itself and that's it.
What we did (I did, years ago) was to "get into the internals of Android code" - and when Google made some changes there in 7.0, this stopped working and we can't do anything about it on message view, it's really Google Chrome messing up the "fragile hack" that's not supposed to be there in the first place.
And so, to avoid further complaints, we removed the "custom language" setting if the device is running Android 7.0 or newer.