We are using an online tool for site translations which is quite easy to navigate and get started with. To start translating you can visit
http://translate.project-gc.com/ . You can either create an account at the site or use Twitter/FB/G+ to login.
If there are any strings you do not understand, an issue can be created at the translation site. We will then be notified and can respond to it. By keeping the discussions on the translation site we will help the translators for the other languages to understand it as well.
In some cases you will find strings containing %d or %s. %s will be replaced by a hard coded string. It might be a site name, user name or some html. %d will normally be replaced by a number and the sentence might exist as both singular and plural. Both variants will need to be translated. The web interface will help you with that.
Some strings contain small bits of HTML, try to keep it intact. It's usually very short tags and they should be easy to understand.
Don't miss to click the Settings wheel while translating to find some great keyboard shortcuts.
As it works today, translations created are manually added to Project-GC, therefore there can be a few days of delay. Usually depends on the total amount of translations that has been made recently.