Full Disclosure: I've never written a mobile app in just one programming language. Yeah, mobile apps are typically targeting two different platforms, so this kinda makes sense. But it still seems like with all the technology and game engines available that you would be able to write in just C++ (or what have you), after all it is the 21st Century.
In my previous rant, I talked about how I've released games (and apps) using cocos2d-x and how awesome it was to just write majority of the code one time. That is still true, but I've also worked on projects that require some feature that is not yet supported in engine or a team that insists on doing everything native. By doing this you have a single app that encompasses three different friggin' languages (C++, Java, Obj-C/Swift), so I am dubbing this the "Friggin' Franken-App". But is the Franken-App the nightmare it sounds like or is that the reality of mobile development?
Sometimes you are brought into a project long after it has been started, so you either have to go with the flow or put development on hold for two months and fix the cryptic, multi-lingual mess. Ever since Swift has come out everyone wants to use it, but not instead of Obj-C, in addition to! WTF! So now Friggin' Franken-App has an extra language to unnecessarily complicate things. This means that one app can have 4 different languages....really! I'm not kidding here. You have C++ to handle most of the cross-platform code, then on occasion you have to add a native notification so fine add Java and Obj-C to handle that. But why Swift? If Swift, then why Obj-C too? Just pick one, not both! My rant about Swift will come at a later date, but for now lets just stick with one language for iOS please.
Even if you chose just one language for your game you might still run into a nightmare-ish problem. Imagine writing an entire game in Obj-C, published and live on the app store. Then you find out that your client wants you to publish it to the Google Play Store as well. This is where the Friggin' Franken-App would have come to the rescue. This isn't to say you have to know the ins and outs of every language of your Franken-App. When you go to a Mexican restaurant you only need to know that it is "torti-ya" not "tort-til-la". From my personal experience of having to re-write an entire game, trust me...it's not fun.
Frankenstein was a nice guy, he was just scary looking. So maybe the Friggin' Fraken-App isn't such a bad thing, it just sounds scary. The reality of it is that you have a few different languages in the game: most of the code written in one language, some features written in two different languages, but in the end you have a game that can easily port.