Set parent theme language with custom .mo files

I think you can just move the languages folder to /wp-content/languages/ then you prevent the theme to update your translations of the theme. And then you can add your own little plugin to change the load_theme_textdomain path to the new folder in wp-content:

Add this as a plugin in /wp-content/plugins/buttercream-lang.php

/**
 * Plugin Name: Buttercream Custom Language
 * Plugin URI:  https://github.com/pontusab/buttercream-custom-language
 * Text Domain: buttercream
 * Domain Path: /wp-content/languages/
 * Description: Add your own languages to Buttercream without losing when update.
 * Author:      Pontus Abrahamsson <[email protected]>
 * Author URI:  http://pontusab.se
 * License:     MIT
 * License URI: http://www.opensource.org/licenses/mit-license.php
 */

add_action( 'after_setup_theme', 'wpse_load_languages' );

function wpse_load_languages()
{
    load_theme_textdomain( 'buttercream', WP_CONTENT_DIR . '/languages' );
}