Use translations from #WordPress plugins first

glotpress-last.php

<?php
/**
 * Plugin Name: GlotPress Last
 * Plugin URI: https://www.screenfeed.fr
 * Description: Prevent WP from loading a plugin's .mo files from the languages directory first. Instead, WP will first try to load the files from the plugin.
 * Version: 1.0
 * Author: Grégory Viguier
 * Author URI: https://www.screenfeed.fr/greg/
 * License: GPLv3
 * License URI: https://www.screenfeed.fr/gpl-v3.txt
 */
if ( ! defined( 'ABSPATH' ) ) {
	die( 'Cheatin\' uh?' );
}

global $wp_version;

if ( version_compare( $wp_version, '4.6' ) < 0 ) {
	return;
}


add_filter( 'load_textdomain_mofile', 'sf_plugin_translations_first', 10, 2 );
/**
 * If we try to load a .mo file from wp-content/languages/plugins, return the path of the file from the related plugin instead (if the file exists).
 *
 * @since  1.0
 * @author Grégory Viguier
 *
 * @param  string $mofile Path to the MO file.
 * @param  string $domain Text domain. Unique identifier for retrieving translated strings.
 * @return string         Path to a MO file.
 */
function sf_plugin_translations_first( $mofile, $domain ) {
	$langs_dir = WP_LANG_DIR . '/plugins/';

	if ( strpos( $mofile, $langs_dir ) !== 0 ) {
		// It's not a file from wp-content/languages/plugins.
		return $mofile;
	}

	// We need to grab the path of the folder within the plugin. It's dirty, but it's the only way.
	$backtrace = debug_backtrace();

	if ( ! $backtrace ) {
		return $mofile;
	}

	// The file name with a heading slash.
	$file_name = str_replace( $langs_dir, '/', $mofile );

	foreach ( $backtrace as $trace ) {
		// We need the 3rd argument passed to load_plugin_textdomain().
		if ( 'load_plugin_textdomain' !== $trace['function'] ) {
			continue;
		}

		// Build the new file path.
		$plugin_rel_path = isset( $trace['args'][2] ) ? $trace['args'][2] : false;

		if ( false !== $plugin_rel_path ) {
			$path = WP_PLUGIN_DIR . '/' . trim( $plugin_rel_path, '/' );
		} else {
			$path = WP_PLUGIN_DIR;
		}

		if ( is_readable( $path . $file_name ) ) {
			// We have an existing file \o/
			return $path . $file_name;
		}

		break;
	}

	return $mofile;
}