To be best prepared for your main launch, a good plan might be to release in a smaller market early (which is called a soft launch) in order to get more data and make any fixes or tweaks prior to your theoretically more important core launch in a major market.
Not all developers can afford to do this, but it's easier if your app uses very little text (or none at all). For example, it's quite affordable to find outsourced localization services that can quickly turn around your app description copy into another language, and be effectively localized.
Chapter 5 provides more information on localization.
Once you're ready with the language and app description/marketing in a selected country, try releasing in one of these markets approximately four weeks (at least) prior to your core marketing launch date. Your app, after all, should already be in a roughly releasable state, and your goal, therefore, will be to concentrate on any unforeseen major bugs, as well as getting as much data as you can for everything else.
An important thing to note for soft launches is that they can save you time to release with a slightly reduced feature set than you are planning on with your main launch. Then you can eventually update all versions to be the same on main launch. In this way, you iterate through the feedback you get, and patch any changes to your ...