I have the exact same problem. It drives me nuts, that I can't turn it off. I have turned off "Highlight misspelled words" in Windows 10 settings, but it still underlines every single word I write in my notes, as I often take notes in different languages. I can't abstract from it, when I'm writing longer texts, so I have turned to writing my notes in Notepad and then copy paste it to Evernote afterwards. That is just counter-productive.
I generally hate spelling aids. Notes are not always 100% percent correct, as the notetaking is often done in haste. I want my notetaking app to be like my Moleskine notebook. Just a blank canvas in most cases.
Evernote, please fix this.