I recently installed Evernote on a Mac laptop, and the spell-checking feature seems to apply only British English and not US English. For example, it flags "organization" and "initialization" as incorrect, and suggests to change them to "organisation" and "intialisation". I can select to learn the US spelling when I use it, but I don't want to do this for each word. I could not find a way to configure Evernote to use US English. I also checked all of the Mac system preferences related to language, and the only English-flavor reference that I could find was in the keyboard settings, which are configured to use US English.
Mac OS version: 10.11.6 ; Evernote version: 6.9
On my Windows 10 laptop, running Evernote v6.2.4, the spell checking uses US English and flags British English as incorrect. I believe this is also true for Evernote on my Windows 7 work laptop. I am using the latest Evernote versions available for each platform.