/var/www/openmediavault/js/omv/globals.jsを編集します。配列の最後に日本語を追加するだけです。一応全部掲載しますね。
/**
* This file is part of OpenMediaVault.
*
* @license http://www.gnu.org/licenses/gpl.html GPL Version 3
* @author Volker Theile <volker.theile@openmediavault.org>
* @copyright Copyright (c) 2009-2013 Volker Theile
*
* OpenMediaVault is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* any later version.
*
* OpenMediaVault is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with OpenMediaVault. If not, see <http://www.gnu.org/licenses/>.
*/
/**
* @defgroup webgui The WebGUI Application Programming Interface
*/
Ext.ns("OMV");
OMV.languages = [
// The supported languages are hardcoded and are not translated.
[ "en","English" ], // English
[ "de_DE","Deutsch" ], // German
[ "ru_RU","Русский" ], // Russian
[ "nl_NL","Nederlands" ], // Dutch
[ "it_IT","Italiano" ], // Italian
[ "el_GR","ελληνικά" ], // Greek
[ "tr_TR","Türkçe" ], // Turkish
[ "fr_FR","Français" ], // French
[ "pl_PL","Polski" ], // Polish
[ "es_ES","Español" ], // Spanish
[ "gl","Galego" ], // Galician
[ "hu_HU","Magyar" ], // Hungarian
[ "da_DA","Dansk" ], // Danish
[ "uk_UK","Українська" ], // Ukrainian
[ "no_NO","Norsk" ], // Norwegian
[ "sv_SV","Svenska" ], // Swedish
[ "pt_PT","Português" ], // Portuguese
[ "zh_CN","简体中文" ], // Chinese (China)
[ "zh_TW","正體中文" ], // Chinese (Taiwan)
[ "ja_JP","日本語" ] // japanese
];