I have several iOS/iPhone apps that have been continually selling in small amounts in over 2 dozen different countries, even though the app UIs and all the store descriptions are only in English. In a few countries where English is not the official or native language, a few apps are selling far better than is proportionate for those country's population size compared with the U.S.

So why Internationalize apps?

What kind of increase, if any, in sales might a typical app see if it is Internationalized into given local languages? Which major languages might be likely to see the greatest improvement in app sales or downloads due to a localized app description?