1. Go to "Review" on top of the document (next to Mailing and View)
2. Select language, Select Language again. Select Set Proofing Language...
3. It gives you a list of languages to chose from. Select the one you are looking for.
4. Do not select Set as Default since you said you just want it for this particular document.
5. Click ok.
6. To proof that you added a new language: Select Review again, Select Language and select Language Preference instead. You will see a list with the language you just added. You can acquire more languages by doing the same steps 1-5. One of them should say default, I'm assuming, it will be English since this is the first time you are adding another language.
7. Turn off the computer and restart it, so your changes take place.
8. Before you start typing your new document go to Review, go to Language, go to Language Preference and select the new language. Start typing.
Since you have other languages on the list, when you start typing on another language (English) the machine will mark it in red momentaneously, but it will erase the red marks when it continues recognizing the same language over and over. This is because your new language is not a default at this time. If you have it as Default anything you type from another language it will mark in red and it will stay that way until you fix the words.
9. I recommend when you finish typing your document, highlight everything, go to Review, go to Language, go to Language Preference and select the new language again to remind the computer you want this language. It will mark in red your foreign words, if any, or any misspelled words and you can fix those.
I hope this will help you to answer your question. I was doing it at the same time while I was explaining to you, to be very precise and accurate. It worked for me, so it should work for you. I'm not a Word Teacher. I have tutored before, but I just know the basics.... because I use it a lot for my Spanish handouts. I have used Word for over 20 years. I should be an expert but nahhhh.... I'm a Spanish professor, that's my specialty. 😀