For completeness, there are 3 ways to add or modify translations in a different language. Your question relates to the third method.

1) Work on Launchpad using the dedicated Translation interface. This is automatically synchronized with the .po files of the source code. This is the best option because your work is visible to the community; you get suggestions based on terms that other software has already translated; you don't need any programming skills; and others can easily work with you.

Web: https://translations.launchpad.net/openerp-web/7.0

Addons: https://translations.launchpad.net/openobject-addons/7.0

Server: https://translations.launchpad.net/openobject-server/7.0

2) Work directly on the PO files. You can edit these directly using POEDIT. You will generally find a I18n folder in each module. The folder contains two type of files .po and .pot. The .po files are the actual translation files where as .pot is the template for the translation. The .po files should be named according to the language code of specific language and .pot should be named according to the module name.

3) Export, edit and import terms via CSV. Settings --> Translations --> Import / Export.

More information at http://doc.openerp.com/v6.1/contribute/07_improving_translations.html