I have this exact problem too. It's not about automatic 'correction' but about the underlining.
Basically, I use a lot of different languages and 'rare' words etc. in a document so that things get underlined (non of them mis-spelled).
It's confusing, distracting and just generally annoying that a company forces a - in this respect - stupid computer on me while I work.
As of now, I have to all the time go to Edit -> Spelling and Grammar to tell the respective program to be quiet. Underlining is automatically enabled again and again after coming back to an app. This costs a lot of time and nerve.
A Terminal command or the like would be appreciated.
(P.S. It is fine to spell-check a written document when I, as the user, invoke that feature).