Claudiu Costin has built scripts for a daily documentation statistic. This page gives an overview of the existing documentation for your language and the status (i.e. outdated, current...). This statistics does currently contain only documentation that was at least at some time completely translated — simply because this statistic does compare DocBook files. These files are generated by po2xml only if the corresponding PO
file contains no fuzzies and untranslated strings.
If you have the l10n sources for your language on your computer then the Catalog Manager of KBabel 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.