Add Korean locales

Summary:
Ref T9285, add Korean locales per epriestley.
Korean have no PLURAL/GENDER normally, but plural left since
a little strings may need some plural expressions.

Test Plan:

• Have a Korean translation file.
• Go settings, no Korean in language selection
• Apply Patch
• arc liberate, and voila! Now Korean (Republic of Korea) is there

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: seungrye, Korvin

Maniphest Tasks: T9285

Differential Revision: https://secure.phabricator.com/D14228