BUKKIT-2220 Player.getLanguage()

This pull request adds the method getLanguage() to the Player interface to retrieve the Player's configured language.

Use cases:
- per-player translation
- language statistics

As discussed on IRC: This doesn't fix the lack of a proper localization system, but it allows the plugin developers to implement their solutions untill Bukkit provides one, without depending on reflection crap. And even if Bukkit finally adds a localization system, someone might still implement it in a different way as it might suit his project better. I planned to open-source my implementation as a small independent library.


Addresses [BUKKIT-2220](https://bukkit.atlassian.net/browse/BUKKIT-2220)

CraftBukkit PR: https://github.com/Bukkit/CraftBukkit/pull/889