Before installing and enabling the needed language packs for VirtueMart, you should first install the chosen language packs for Joomla.
For Joomla 1.5 language packs are installed from the admin area -> Extensions -> Install/Uninstall. You just have to download them from:
http://joomlacode.org/gf/project/jtranslation/frs/
For Joomla 2.5+ language packs can be easily installed with the Extensions -> Extensions Manager. Just go to the Install languages tab, select the desired languages and click Install in the upper right corner.
Then, you should upload the corresponding VirtueMart language pack files in the administrator/components/com_virtuemart/languages folder.
VirtueMart language packs can be found in the application’s extension store at: http://extensions.virtuemart.net/index.php?option=com_sobi2&catid=7&Itemid=2
After that, change your Joomla’s default language from the Joomla administrator area -> Extensions -> Language Manager. This way VirtueMart will use the newly set language by default and update itself.
*Note: the language files for VirtueMart and Joomla should have the same names. Alternatively, the correct value, in Joomla 1.5, for the backwardLang tag in the XML installer file of the language should be set.