There is no support yet for custom languages or more languages.
This method is in the works though that would allow you to write your own auto-translation through the localization spreadsheet: http://wiki.roblox.com/index.php?title=API:Class/Translator/Translate
(does not work yet last time I checked)
I would just wait for Roblox to release a better mechanism for supporting more languages though. Above is too much work for a very temporary solution.