I’ve downloaded the original language .mo and .po files translations (it_IT) from the official repository https://github.com/roots/sage-translations.
I put them in the theme-folder/lang/, and added load_theme_textdomain( 'sage', get_template_directory() . '/lang' ); inside the after_setup_theme function in setup.php.
But the translations are not loaded (for example when going to the 404 page), I still see the english strings.
I tried to disable the Polylang plugin, but the issue remains. I didn’t change the text_domain from ‘sage’.
My problem seems different, or it’s my fault and I didn’t get how to solve it.
Wordpress seems to totally ignore the lang folder inside the theme one (even if I define it in setup.php as stated above), but it loads the web/app/languages/themes/themename-IT_it.mo.