I think you mean not "convert" a .po file into another language, but to actually translate its content, right?
If you're speaking about a commercial project, then your direction is toward freelance interpreters, which are plenty around the Internet for a vast majority of native languages.
If you're creating a Free (Libre) and Opensource project (you've answered that question)
I think you should ask for help in this software-related forum. Maybe, create a topic about your project here. As far as I've seen by now FOSS programs have a much wider support for native languages (however, not always trnanslations are of high quality).
I've been using Transifex for some time, and it's relatively user-friendly, automatically exports-imports .po files. Of course, an interpreter would want more power, but it's widely used and has a not-very-limited free version. Many FOSS projects such as FreeDroidRPG and SuperTuxKart use Transifex as far as I know.