First, you must download the source code of SOGo; either the latest release or the latest revision from the git repository.
Three type of files must be translated. The first type are Localizable.strings; they can be translated online using Transifex. To use Transifex, you must first sign up for a free account. Once registered, request a new team for your language. Once authorized, you'll be able to start/continue translating SOGo in your language.
The second type are wox and html templates. Only words outside the tags (<>) must be translated. Start by duplicating the English templates: