Tired of your custom plugin and theme translation files getting wiped out every time you use the WordPress auto-updater? Install and activate this plugin and
WordPress will look for missing translation files in the WP_LANG_DIR where you keep your WordPress translation files.
In WordPress 3.8 and greater this plugin’s functionality is included in core. You can put translation files in:
* Plugins: /wp-content/languages/plugins/textdomain-locale.mo
* Themes: /wp-content/languages/themes/textdomain-locale.mo
A free plugin by Aaron Edwards of UglyRobot Web Development.