[qualiteam]

In Twig templates you should use the t() function:
http://devs.x-cart.com/en/basics/twig_guide.html#t

In PHP classes you can use the static::t() method. For an example, see this article:
http://devs.x-cart.com/en/getting_started/step_4_-_working_with_settings/creating_custom_setting_class.html