As a Dutch-man, I almost always write my emails in British or American English. And do appreciate a good spell-checker.
Both English spell-checkers underline way too much words as being wrong while they aren’t. I am quite tempted to shut them down, only have to find out how.
Please consider allowing Grammarly as an add-on.
Another thing is: mails can be partly in one and partly in another language. Currently, Emclient thinks it is entirely in one language, a bit simplistic. Excusé le mot, s’il vous plait.
Is it possible that the OS on your computer is in Dutch? If so perhaps this could be the cause of your issue. Mty OS is Spanish and I had and still do have occasional issues with words that I have not added to my personal dictionary (right click word & select add to dictionary) It’s not great but it works and gets less and less over time.
I believe Grammarly used work with eMC, however, there have been recent posts concerning a compatibility issue. I do not know if it needs to be resolved by eMC or Grammarly or both. I have never used Grammarly as I do not have a need.
If the OS language had anything to do with it, it would be even stranger. But for example is every first word marked, it seems to me it can’t understand that a capitalised word can be correct too.