The language in which the whole ViArt Shop operates is selected during the installation procedure. However, if you want to change it on a later notice, you have to edit 'var_definistion.php' file which is stored in the 'includes' folder of ViArt Shop.
Locate the following string and change 'en' to an appropriate language code.
$default_language = "en";
Note: Correct language codes are stored at Settings > Static Tables > Languages > Language Settings.