The spellchecker in Word on a Mac is using a different language than what is set for the system, program and dictionary.
The language in OSX is set to English, the language in MS Word is set to English, and the spell checking dictionary in MS Word is set to English. Yet when I open a particular document (which was written in English, though I don't know what language the person who created it uses on their computer) and try to spell check it uses a Dutch dictionary and there doesn't seem to be a way to change that.
Even if I open a new document and type something to verify the spell checking is in English and then cut and paste from the other document, it then uses the Dutch dictionary to spell check.
It happens on only one specific page in the document.