Here's a quick tutorial from Steve Young on how to localize your app for the App Store. This is different than localizing the code and design of your app. For example, you can localize your keywords, description, and screenshots only. This would be your first step to fully localizing your app. You could even use this as a test to determine if localizing your app is worth it. Try this and monitor your downloads.
There are some things to consider, Steve points out...
The downside is if your app’s content is not localized, then you may get a few angry customers who feel you mislead them. If that’s the case, then it’s important to only localize the text outside the app and not the text within the app.
How to Localize Your App and Increase Downloads by 200% by Steve Young