English (UK) mac users still have a problem with this as at January 2021 - my sympathies to everyone else as well. 'Overtrousers' and 'pajamas' are both recognised when typed into Google Search, but not in Evernote. Red underline, no ability to turn the spellcheck off and no spelling dictionary to add valid words to. This is the story with the 'latest' version: 10.6.9-mac-ddl-public (2254), Editor: v116.1.14913, Service: v1.26.7 - which I was offered and loaded mid-January 2021.
By the way - if there is the ability to add words to a dictionary, there also needs to be the ability to remove them from there... basic data life-cycle stuff... just saying, so when it does arrive it works properly...