Hello,
1, 4/ Translation of XStore Core plugin. Go to Loco Translate > Plugins > select the XStore Core plugin. Press “Add new language” the same way as you did for the theme translation. Translate the necessary strings and Save.
Custom CSS code https://prnt.sc/1vjcn2t
2/ Translation of the theme and Core – https://xstore.helpscoutdocs.com/article/30-base-theme-translation
3/ To enable registration, navigate to Settings > General > Membership and mark Anyone can register, then go to Woocommerce > Settings > Accounts > Customer registration > Enable customer registration on the “My account” page.
Read more about settings – https://docs.woocommerce.com/document/configuring-woocommerce-settings/#accounts-and-privacy-settings
FaceBook login – https://xstore.helpscoutdocs.com/article/87-facebook-login
Regards