I started this game at April, and it took me 4 months to get through the whole story line and reach LV 60.
From the perspective of a non-native speaker, that long cutscene surely helped to improve one's reading ability, but only reading(or I'd rather say, understanding).
As what others say, the dialogue style in this game is outdated. You might expand your vocabulary here, but had better not follow the way they speak.
A better way to practice one's language, as I found after LV 60, is joining team play with native speakers, or just staying here--the official forum--to discuss with others.

I find even through a game, the way of learning a language does not change a lot: you should learn it by using it.