How would you like to ask? be spontaneous !!!! be you... =D (I'm assuming you are kind, polite and respectful ^^ )

I would always greet them (hello, how are you?) in their language, and give a brief message why you want to add them as a friend that you're interested for learning their native tongue. Just don't add random people that you see and never talk to....

Hi... Just send a "friend request" if you don't know what to write......

would you be my friend?