The page for a daily documentation statistic gives an overview of the existing documentation for your language and the status.
If you have the l10n sources for your language on your computer then the Project Overview of Lokalize contains considerably more information.
Another important point is to not duplicate the work of somebody else. It may be useful to split the work so that for each package one person (and only one) is responsible. Usually, it is the job of your language coordinator to organize this part. So you should better not start to work before you have checked with your language coordinator.
There is another item to check for: Before translating you should contact the author of the English original documentation to make sure there is not going to be a major new revision in the near future. By doing this you make sure your work is not obsolete.