The following convention is used for naming the translation files:

LANGUAGE_COUNTRY.js

Where:

LANGUAGE = The lowercase ISO 639-1 language code.

See http://en.wikipedia.org/wiki/List_of_ISO_639-1_codes

COUNTRY = The uppercase ISO 3166-1 country code.

See http://en.wikipedia.org/wiki/ISO_3166-1_alpha-2#Officially_assigned_code_elements

When there is no COUNTRY in the filename, it is assumed the generic language is used in multiple countries.