Change the user language depending on the username. This example defaults to Japanese, unless you are me

change_user_lang.php

<?php
/**
 * Plugin Name: Change User Language
 * Description: Change the language depending on the user
 * Version: 0.1
 * Author: AJ Zane
 * Author URI: http://AJZane.com
 * License: GPL2
 */

add_filter( 'locale', 'change_user_lang' );
function change_user_lang( $locale ) {
	//Define the user list using english
	$users_list_en = array(
		"ajzane"
		);
	if ( in_array( wp_get_current_user()->user_login, $users_list_en , true ) ) {
		return 'en-US'; //or whatever you want your native language to be
	} else {
		return 'ja';
	}
}