I don't know that I'd call the main NPC dialogue unintelligible (aside from Urianger, whose dialogue is hilarious). They just take really roundabout ways to say stuff.

Instead of saying, "He punched me in the face," they would say, "He dealt a swift and mighty blow to mine countenance," which I guess adds to the fantasy setting, or something.

More to the point: interaction with the playerbase will help your friend learn English. The game itself likely will not.