I often write in a mixture of Scots and English, and spellcheckers are nae guid for that use case. The problem is solved, however, on iOS and Android thanks to SwiftKey[1].
[1] https://dsl.ac.uk/our-publications/scots-predictive-keyboard...
[1] https://dsl.ac.uk/our-publications/scots-predictive-keyboard...
Honestly I can't use the default keyboard in iOS anymore because it lacks multilingual support. How it still isn't a thing in 2020 boggles me.