How to update language files - Plone translation files
Today, there is a product - PloneTranslations - which contains all recent i18n files (*.po) for Plone. Starting with Plone 2.1, PloneTranslations ships as its own Product (while still included in Plone releases, of course).
http://plone.org/products/plonetranslations/
To update your translations: download and unpack the product, and replace all the .po files from the old product with the ones from the new. Restart zope.
The .po translation files are kept inside PloneTranslations in its i18n folder
your_instance/Products/PloneTranslations/i18n

