[B+C] Expose locale to plugin developers. Adds BUKKIT-4507

##### The Issue:
The API currently does not allow plugins to get the player's language setting, thus preventing plugins from implementing language support.

##### Justification for this PR:
Plugins will be able to detect the player's locale setting and adjust sent messages accordingly, allowing player-specific messages in their own languages.

##### PR Breakdown:
The PR provides a getter for locale in the Player interface.

##### Relevant PR(s):
B-#### - link - Add the getter to the Player interface.
CB-1192 - https://github.com/Bukkit/CraftBukkit/pull/1192 - Expose the locale variable and implement the getter in CraftPlayer.

##### JIRA Ticket:
https://bukkit.atlassian.net/browse/BUKKIT-4507