Aaron Edwards

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.
<h4>UPDATE</h4>
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 <a href="http://uglyrobot.com/" rel="nofollow ugc">UglyRobot Web Development</a>.

Translation Preserver

A simple plugin that helps preserve your custom plugin translations through auto-updates by allowing you to put them in your WP languages folder.

Seisuke Kuraishi

Multibyte functionality enhancement for the WordPress Japanese package. 
<a href="https://eastcoder.com/code/wp-multibyte-patch/" title="Documentation in Japanese" rel="nofollow ugc">日本語の説明を読む</a>

WP Multibyte Patch

Great!

いつも大変お世話になってます！ありがとうございます！

Thanks You!

Great plugin

Multibyte functionality enhancement for the WordPress Japanese package.

Translation Preserver vs WP Multibyte Patch