How can I translate the checkout system?
In importing translation page /admin/config/regional/translate/import I have chosen "Drupal commerce"in Text group section. The process went fine, but the words to translate were indeed in Bult-in interface. So I imported the same file again but now choosing "Bult-in interface" in Text group section. Now I have checkout system translated at some degree.

I think perhaps because I installed Commerce Kickstart 2 and Drupal Commerce comes all in Built-in interface.