<p>I have developed a web application using internationalization best practices such as putting all my displayable strings in property files, etc, etc.</p>

<p>I would like to have the strings in the property files translated into 5 different languages.</p>

<p>Does anyone have any experience using Mechanical Turk or another crowd sourcing service for language translation?  </p>

<p>The reason I don't want to just hire a translation company or service is because I want to eventually have ongoing content fed into the translation service via an API.</p>

<p>My Google results for more information on this topic were surprisingly dismal.  Any links or pointers are appreciated.</p>

I don't have any experience crowd-sourcing translations, but my advice would be to find some dependable freelance translators in your target languages (check out Proz.com), and keep going back to them every time you have new content. Application UIs are notoriously difficult to localize because of lack of context in the resource files, and so you want someone who understands the application and is able/willing to test the localized version. Each of the localizations will undoubtedly reveal i18n bugs as well, so there is likely to be a bit of back and forth the first time around. I guess my point is that you don't want arms-length relationships with your translators; they should feel like an extension of your team.
Anyway, bravo for internationalizing your application. Good luck!