An idea - maybe there is a chance to make a plugin for Mailspring that would use LanguageTool API documentation ?
I’m not a programmer, so I have no idea if it is or isn’t possible to create a spell checker plugin 
However, I have seen the obsidian.md plugin, so I assume it is possible.
PS I see topic about here: Language Tool Addon
But silence.
Really one of best email clients, which I tested, but this issue is making me step of uninstall it and forgot. How to pay for soft without this basic function?
However, I hope that this can somehow be fixed soon.