Where I can change the string: (ART_VIRTUEMART_CART_EMPTY)

There are many language packages available for Joomla but the template has some custom strings of text. Don't worry though, translation is really simple. All language strings are stored in the language/xx-XX folder, where x is the language key (e.g. en-GB for english and es-ES for spanish). You need to copy en-GB.tpl_t3_blank.ini (For Flatastic:en-GB.tpl_t3_bs3_blank.ini) to your language folder and change the language code in the name. Then open it with any text editor and translate the few phrases/words in the "frontend strings" section.

For login copy en-GB.mod_login.ini

for cart en-GB.mod_virtuemart_cart.ini

for wishlist en-GB.com_vm2wishlists.ini

for compare en-GB.com_comparelist.ini

for tags en-GB.com_vm2tags.ini and etc