Found a solution that works for me:
add_action( 'after_setup_theme', 'avia_lang_setup' );
function avia_lang_setup() {
$lang = apply_filters('parent-theme-slug', get_template_directory() . '/lang');
load_theme_textdomain('avia_framework', $lang);
load_child_theme_textdomain( 'child-theme-text-domain', get_stylesheet_directory() . '/lang' );
}
Note:
Exchange “parent-theme-slug” (typically the parent theme name in small letters) and “child-theme-text-domain” (child theme text domain of your choice)