The English used in the game's text is pretty old-timey, if it is also littered with "flavour" like the pirate dialect in Limsa, it can be really hard to understand anything at all, especially to someone who is still learning (I know at least I dropped my attempt to play in English because of that..).
The English the players speak is often very faulty, either because they also aren't native speakers or ARE native speakers but - because it's the internet - appearently don't know how English works anymore ^^°

Overall, I wouldn't call the game a good place to learn the language. But people learn the fastest when they do something they enjoy, so...yeah.