You might just try copying the language file and dropping it in the main language folder. In the old days it worked!
Here are my notes on creating a custom language pack, but I will tell you that they are really for 1.8 and should be getting obsolete.
Here are a couple of other interesting links; maybe they will help?
http://docs.moodle.org/en/Installation_FAQ--look for "Why doesn't my Moodle site display the time and date correctly?"
http://docs.moodle.org/en/Language_packs--(How to install a second laguage pack)