Is it possible to change the display language as a preference for the forums? 

Nope, not as the forum is set up now at least - U.S. English only. But other languages may very well be available if enabled.

But why would you like to change it? All the replies are going to be in English anyway, so even if Danish were available (my native language) I wouldn’t choose it.

 I have spent 3 1/2 years in French classes, and a year and a half later, I'm forgetting a lot of it. I've changed a lot of my regular websites' preferences to display everything in French so that I see it a lot and get to use it even while I'm out of French classes and not able to use my knowledge of French daily in conversation. It would be fabulous if I could get BAF to be displayed in French, too (at least the messageboard, anyway... the website would take WAY too much work for that). Sooo I was wondering if there was some sort of preference for the forums that could be installed to show other languages (the same way that there are points, different colors, etc... something that came from the forum provider, not something that BAF would have to spend a lot of time on or anything).

Shouldn’t be a big deal. Whoever is maintaining the forum just have to install the French language pack, set U.S. English as default, and then you should be able to choose French in your profile editor.
But maybe they don’t want to/have the time to mess around with it.. Can’t imagine that it is something a lot of users would have any use of.

http://docs.communityserver.com/faqs/does-community-server-support-multiple-languages/ 

http://dev.communityserver.com/media/p/552173.aspx