As a localization professional, I work with a lot of developers on their apps and always ask questions like this. In truth, it varies considerably because apps gain traction in different countries almost at random. Well, not really random, but it has a lot to do with whether a blog or foreign language post talks about the app, giving it that first nudge up the charts. This isn't all too different from English-language apps, you know!
I always recommend for developers to localize in languages where they SEE (in iTunes Connect) people already downloading. Notice an uptick in downloads from Germany? Localize into German and you'll see those numbers take off. However, translate it into French when zero users are currently downloading it, and you may or may not succeed. It's not just about numbers, it's about momentum.
Generally, the most successful languages are: Spanish, Japanese, Korean, French, German, Portuguese, Simplified Chinese, Italian.
The bottom line is: if your app isn't successful in English, don't localize it. That won't change anything. But if your app is successful, start in one or more of the languages above and see where it takes you!
I wrote a blog post about this in more detail here: