Selling apps in multiple countries increases your odds of making money, but how can you translate all the interface elements? Ackuna lets you upload app project files and have them translated by volunteers.
The idea is that as well as having your apps translated by others, you'll volunteer your own services to help out fellow developers. The service uses a reputation system, which should (at least in theory) ensure that you don't end up with your app translated into a series of swear words. The elements you want translated can be submitted in a variety of formats, including YAML, spreadsheets and specially-formatted files.
If you want a truly professional polish on an international app, a paid translator is arguably still your best alternative. However, for smaller project and prototypes, Ackuna looks promising.