Changing the language doesn't change the system spell checker
AnsweredAs I am typing this on my Swedish phone with the SwiftKey language set to English, I get red squiggles on every word that isn't Swedish. Using the Google keyboard and switching languages doesn't cause any problems, even when using multiple languages in the same textbox.,
This is because Gboard has advanced system privileges that SwiftKey cannot access.
Sadly, there is nothing we can do on our front to control this behavior.
Thanks for the explanation! Google having special privileges, who would have thought.
Going from Windows Phone to Android is an exercise in frustration and despair, I can tell you that.
But what about other keyboard apps, for example Yandex.keyboard? Why

there are no problems with them? There are 2 screens with SwiftKey and Yandex Keyboard. SwiftKey did not switch spell checker, Yandex did. Please fix this it can be fixed.
I struggle to believe the answer is correct.
It underlines anything that is not English (the UI language) on my phone (three or two languages), but I've seen it working on my colleague's phone (two languages). It seems like his spell checker can somehow handle two languages. How come??
Are there any news on this front? Looking at the replies, it seems like this is not impossible at all. Please, fix fundamentals like this first before adding stickers and whatnot.