1. Language is your means of communication to the user
Be aware that your interface design and language go hand in hand. They form one single entity and must not be torn apart. Your interface is the map that guides the user to their destination. Now imagine what happens if the user does not understand what's written on this map, as pretty and intuitive as it may be...
2. Your user needs to understand your intention
If a user does not understand how to operate your application and what the application actually does, you have failed. This scenario is much more likely when having to deal with a foreign language. Your UI might be damn well thought through, but if the user does not understand the function of a certain button or option, your whole app is going right down the drain.
3. Only a mother-tongue UI is truly intuitive
Would you know what a button called "Inoltra" is good for in an email application? If you don't speak Italian, you probably won't. The same applies to many potential users who don't or hardly speak any English and would like to test and use your app. Don't let them sit in front of their displays clueless as to what they could do with your great app.
4. Localization is an added value
Check out your competitors. Do they offer localized products? If they don't, well there you go. Show foreign customers that you actually care about them. They will most likely kiss your feet (or whatever) and turn down any competitor's app for an app in their native language. What if your competition does offer localized versions? Well, they probably have a major competitive advantage on you in foreign markets. You might want to fix that, too.