Jerome Gotangco wrote recently in his blog that he's translating Jokosher into Tagalog (and later Filipino) using Rosetta.
This is a good track to take if you are interested in getting more applications translated into your language of choice.
Choose an application that you like using, use often, and that you are familiar with.
Work on translating the interface of that application. It will involve some studying because a haphazard translation will indeed look funny.
Choose an application with a workable number of messages to translate. This will make your target of translating it more attainable. Otherwise, it will be a daunting task. Set aside time to work on translating a handful of messages everyday maybe.
Using Rosetta to leverage team translation is useful. The web-based interface makes it simple for anyone who is interested in contributing translation work to get involved. It helps make everyone else on the team aware of what others have contributed in the translations.
However, set some objectives for yourself before you start translating. This will help temper your self-criticism of your translation. Remember that translating free and open source programs is slightly different from translating a book or a poem in that f/oss programs usually get updated a lot, thus, you may have to work on updates of your translations from time to time. This means that your translation work is not set in stone. You are free to revise and improve your translation, in much the same way that the program itself is being developed. Your translation can also be the basis for other translators work.
Review the results of your translation if you can, by downloading the .po file and compiling it and then running the program with your locale set up properly. You will be able to see if your translation works only when you run the program in your language. Otherwise, you will be beset with strings that do not make sense because they were translated without understanding the context that they appear in.
By the way, our friends who helped translate Mandrake Linux to Tagalog/Filipino did a fairly good job translating XMMS. It wasn't a perfect 100% translation, but it is pretty close. Kudos to Arys and Renoir Deloso for their translation.