In general Mailspring is pretty good in understanding which language is being typed in order to perform spellchecking, but it needs some amount of text to do this. However, when the text is short, I have found myself without spellchecking and without being able to define one manually.
I’m writing on Mailspring in 3 languages, 2 of which are English and French. In my case this can be a problem as they are not my primary languages and spelling-wise they are close and some times identical, making mistakes easy to happen.
So the suggestion would be to include a spellchecking button in the compose window (that would be in auto-detect by default) that would allow us to force a specific spellchecking language for the specific window when the text is too short or somehow things fail.
Perhaps another way to estimate/detect the language very early would be to sniff the current keyboard language. eM Client employs that method and it has worked very well for me.