How to override languages files in wp-content/languages/themes with child theme

Since WP 4.6 load_theme_textdomain() (and consequently load_child_theme_textdomain()) will give priority to .mo files downloaded from WP’s online translation platform (translate.wordpress.org). Due to some new code (here, on line 769) these functions will completely ignore your local .mo files if the textdomain is found in the general languages/ directory.

You can, however, use the more basic load_textdomain() function to directly load your .mo file and override strings from the original domain, like this:

$domain = 'textdomain-to-override';
$path = get_stylesheet_directory() . '/languages/'; // adjust to the location of your .mo file

$locale = apply_filters( 'theme_locale', get_locale(), $domain );
load_textdomain( $domain, $path . $locale . '.mo' );