Once you've learned enough of the basic fundamentals of a language that something like playing a game in that language even becomes possible, the best way to get better at it is with regular use, and playing a video game in that language can be a good way to provide that regular use.

It's only in a few places where dialog is mimicking accents (the most extreme being the rogue's guild or anywhere you're talking with goblins) that the dialog in the game gets hard to follow. 99% of the game's dialog is clear modern English, more formal from some characters and more casual slang from others, but clear in either case. What I'd be more concerned with is the horrible English that players themselves often use, riddled with all kinds of errors and weird acronyms in place of actual words.

But gaming in a language you're trying to learn can still provide a lot of valuable practice at it.