I'm Brazilian and I want to change the interface of betheme to Portuguese, in the selection of themes and in the administrative panel of the site and I don't know how to do it.
Please, download and install a child theme, activate it, and then the interface will be translated to the language which is used by the user in the WP panel.
In the child theme package is a folder with translations, which changes texts in the admin panel.
How is the normal theme different from the child theme? do I lose some function with it?
A child theme is useful when you want to modify the theme files without losing your changes after the update, but you do not have to make any changes there.
For your proposals, a child theme will not make any difference except for introducing the translated options.