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.

##### Testing Results and Materials:
A test was ran where the results of getLocale() were printed after the vanilla packet that sets it was received. I then switched through different languages in the client settings, noting that the server would pick up on any locale I changed to.

##### Relevant PR(s):
B-999 - https://github.com/Bukkit/Bukkit/pull/999 - Add the getter to the Player interface.
CB-1310 - https://github.com/Bukkit/CraftBukkit/pull/1310 - Expose the locale variable and implement the getter in CraftPlayer.

##### JIRA Ticket:
https://bukkit.atlassian.net/browse/BUKKIT-4507