It’s not very easy to do it. Read this article from top to bottom: http://help.joomla.org/files/EN-GB_multilang_tutorial.pdf
First, there are twotypes of files – .mo and .po (in your wordpress languages and in your themes)
You could edit the .po file directly with any editor and then use poedit to generate the binaty .mo files.
There a few tricks to get this working for your online store with Joomla.
1. Make sure you have installed Virtuemart 1.1.4 and Joomfish 2.0.4 over Joomla 1.5.20 (it should work with any 1.5 version). System Legacy plugin should be on.