Hi,
regarding the questions above:
1) You just need to install atelier-child.zip just like any other theme, after installing it you need to activate it.
That by itself won’t change anything, but future modifications in the child theme will make it possible to extend/changed the theme functionality.
The child theme zip is inside the big zip that is downloaded from Themeforest.
2)Inside the child theme create a directory called ‘languages’ and place there the language files .po and .mo
Inside the functions.php add this line of code
load_child_theme_textdomain( 'swiftframework', get_stylesheet_directory() . '/languages' );
Hope it helps.
-Rui