Any way to get the characters in Magicka to speak English?

The characters are already speaking English. Well, OK, they're speaking some English. Or at least nonsense with a few English words thrown in. Anyway, that's the only language spoken in the game (expect for the between-level narration).

As to what language the characters in Magicka actually are speaking, the developers were asked this in this YouTube video from 2011 (at about 4:55):

Host: We've got a lot of people asking about the language in Magicka. Is it really Swedish?

Dev #1: No, that's ridiculous, Swedish doesn't sound at all like the talking in this game.

Dev #2: Well, it's certainly... inspired by our native language.

Dev #1: Yeah, we actually just tried to, uh... what's it called... wang it? (confusion) Wing it? Yeah, wing it when we were recording, but that didn't really work.

There was also a follow-up question later in the same video series (at 0:30):

Host: What languages were you inspired by when you created the Magicka language?

Dev #1: (Heh.) Icelandic, mostly. (Yeah.) Which is the closest to, uh, ancient Norse.

Dev #2: Initially, yeah, but I think it turned out to be a mix of Swedish, and... strange American accents, here and there, depending on what character we were trying to portray. I don't think the Icelanders, or whatever...

Dev #1 (interjects): Yeah, dude, we added "-ur".

Dev #2: Yeah, the "Fornskogur", yeah, all of the names are very, like, Old Norse style, but, I mean, the language, I don't know...

Another detail mentioned elsewhere in the series (part 2, at 6:20, if you're curious) is that Vlad is the only character in the game with a real voice actor; all the other NPCs are voiced by the devs. As far as I can tell, a lot of what they're saying is just random gibberish, or deliberately mispronounced English words ("se-rye-ously"), with some "creative" translations (like "dunka-dunka" for "party" in the first level; in Swedish, it's basically slang for loud thumping disco music) thrown in for good measure.

Anyway, the upshot of it all is that the spoken dialogue is basically there just for comedy. To actually understand what the NPCs are supposed to be saying (not that it generally matters much), you will need to read the subtitles.

A user on giantbomb.com discovered that they are speaking a english/Swedish hybrid but, there is no way to change to plain english.