The runes used by the Erlkonig are largely based off the Furthak, but are, as many people surmised, simply as direct a transcription from the English as possible from the sources I had. (I lost my reference book, and ended up using a secondary source with a slight variation.)
Runic script is used here for an unidentified language spoken by the magic people. Erlkonig is not translated because A.) I rather doubt he ever bothered to learn other languages, and B.) Even most of the other characters can't understand what's being said. Don't worry, it probably won't come up again in the future...
Jareth and Toby belong to Henson, Lucas, Bowie and Froud.
Javert belongs to Hugo, Boublil, Schonberg, and Kretzmer.
Translation, for the time-pressed/linguistically-disinclined/apathetic/lazy-like-the-artist:
Panel 2.) Careful, you're losing control.
4a.) Having problems?
4.b) You always have to control everything, don't you, Jareth? It would go much easier for you if you just stopped fighting what you are...
5.) How long do you think you can deny the story you have already written?
7.) As you wish. Just don't say I didn't warn you.