tl;dr: "What proportion of Spanish (or some other non-English native) people use English as their phone language?"
I am writing an Android app. I plan to publish it primarily in a specific country. Take Spain for example. Due to implementation constraints, it is a vital dilemma for me, to choose my primary app language; It could be English or Spanish. In Android, usually multi-language apps are handled and translated by Android operating system, meaning that if a person uses English for their phone language, the app is loaded in English, and if their phone language is Spanish, my app will be loaded in Spanish. That said, the developer can force-fix a specific language as the default app language.
This raises a big question: Consider that there are a great number of Spanish people who use English as their smartphone language, but I am 100% sure they would prefer to have my app in Spanish, without having to switch their phone language. Now, to what percent of all the Spanish smartphone users, do this number total to? The answer to this question will help me determine if I should be defaulting English or Spanish as my primary app language.
Put short: Are there any statistics on language choice of non-English-native people for their (specially Android) smart phones?
Any similar (not just Spanish, not just Android) statistics would also help me a lot.