As a non-native speaker of English, I don't think the language used in FFXIV is all that complicated.
Sure, there are some uses of dialectal language, and several cases of archaic language use, but very little of it is hard to understand. Even heavily obfuscated language like the rogues' cant is mostly understandable from context even if you have never encountered half the words before.
I for one enjoy the fact that the language isn't dumbed down to a level where even an uneducated child could understand it, because doing that would lose so many nuances of the writing, and even lose a whole lot of world-building that is shown through how different characters speak differently.
