Another question from Quora:
Babble-on localizes iOS apps and software on a regular basis. What I’ve learned from companies doing it is that they aren’t just guessing. Here are some ways to help you decide which language(s) to localize in:
Go where your users are
For iOS App Store projects, you can look in iTunes Connect to determine where your app is gaining traction. For example, you may see a spike in downloads from Italy — that tells you that localizing into Italian is going to get you even more users, even if Italian is not the most widely spoken language on Earth.
Go where your users will be
Another example is a company that has a good idea for an app that will sell abroad. A client of ours had a beautiful app that showed tranquil background scenes and it was a hit in various countries (so he applied rule #1 and translated into the appropriate languages). However, he then had the idea that a “cherry blossom” scene would do well in Japan. He had us translate the app into Japanese. He was right — he knew users in that country would love it, and they did.
Go for the big ones (that pay)
China has a huge market and translating into Chinese is tempting — but only if you really feel users will PAY for your app there. Otherwise, you are always better going with the most popular: Spanish, French, German, Portuguese (Brazil is a surprisingly good market). Other big localization markets are Japanese and Russian (but again, Russia like China, may not yield paying customers).
One last factor to keep in mind is that simply translating your App Store description can get you a lot of users. It’s cheap to do and has a good result. If your app is not heavy with text, even trying multiple languages won’t cost you a lot.
The return on investment can be huge.