In order to install a new language pack in VirtueMart, it is important to install the selected language pack in Joomla. You can download Joomla language packs from the official Joomla repository.
Once you have the language pack, upload it in the corresponding VirtueMart language pack files present in:
You can change Joomla’s default language through the Joomla Admin area >> Extensions >> Language manager
In this way VirtueMart will work with the new language by default.
It is important to make sure that the language files for Joomla and VirtueMart should have the same file names.
Related: How To Install A VirtueMart Theme?