Hey Olollo,
there are two things that need to be done:
1). in the 4+ versions of WordPress, the “WPLANG” constant has been deprecated; instead, you can use a much more user-friendly “Site Language” setting in the “Settings -> General” admin menu.
2). the localized .po and .mo files should be named according to ISO standards in WordPress – i.e. de_DE for German. You can find more information on localizing WP here.
You can put the language files into the parent theme’s “languages” directory in this case, but if you wish to translate only the child theme specifically, you can follow the instructions here, replacing the “total” textdomain with ‘satori’.
Sincerely,
Andrii / Satori Studio