Actually my mistake, I was overriding the translation of the parent theme using it’s text domain for child theme, while I should have defined a different text domain for child theme and load both domains into child theme’s functions.php like this:
add_action( 'after_setup_theme', 'mr_load_textdomain' );
function mr_load_textdomain(){
//for child
load_theme_textdomain (MR_DOMAIN,get_stylesheet_directory().'/languages');
//for parent
load_theme_textdomain (ET_DOMAIN,get_template_directory().'/lang');
}