A Master's Degree in Modern Languages... What's next?

Ok so I'm thinking about teaching and I can attain a Master's in 3 or so years. However, if I ever want to teach higher level language courses in a university I would need a phd, or so my adviser has told me. There's only one "phd" in modern languages that I know of. So what subject could I get one in? Linguistics? Or do I just have all of this completely wrong? -.-