Put this in any random file inside wp-content/mu-plugins to fix all custom language packs for plugins. Make sure the corresponding custom language packs are in WP_LANG_DIR/pluginName/

functions.php

<?php

// Exit if accessed directly
defined( 'ABSPATH' ) || exit;

function fix_all_custom_language_packs_load_textdomain_mofile( $mofile, $domain ) {
	if ( WP_LANG_DIR . '/plugins' === dirname( $mofile ) ) {
		$mo = basename( $mofile );

		$alt = trailingslashit( WP_LANG_DIR ) . $domain . '/';
		
		if ( file_exists( $alt . $mo ) ) {
			return $alt . $mo;
		}
	}

	return $mofile;
}
add_filter( 'load_textdomain_mofile', 'fix_all_custom_language_packs_load_textdomain_mofile', 10, 2 );