The i18n.json file uses "ISO 639-1":http://en.wikipedia.org/wiki/List_of_ISO_639-1_codes and "ISO 3166 alpha-2":http://en.wikipedia.org/wiki/ISO_3166-1 (region - if applicable):

If you want several aliases for a keyword, just separate them
with a | character. The * is a step keyword alias for all translations.

If you do *not* want a trailing space after a keyword, end it with a < character.
(See Chinese for examples).