I haven't really done much mobile development so I can't really talk, though I think there are a couple of mobile developers that drop by here every now and then...
The primary language Android apps are written in is Java. The primary language iPhone apps are written in is Objective C.
While they are both modern, garbage-collected object-oriented languages, they have a LOT of differences. Porting between them is nontrivial and on such resource-constrained machines it's not practical to automatically port/emulate between them.
While there are examples of languages which have interpreters for both platforms (HTML is one, but while HTML5 and JavaScript are quite powerful, you can't really write "real" apps in them), and while Java isn't "native" code anyway, there's no good solution other than making an application which is sufficiently well-designed and modular enough to be able to port between languages easily. OOP principles and patterns should be portable between any OOP language once you know the syntax and concepts*.
If you're making a game (just a hunch), there are already a number of high-profile game engines which work on both platforms with their own scripting language. If you're talking about an everyday application, you're up the creek without a paddle unless you don't mind taking up both languages.
*ObjC I believe uses some concepts from SmallTalk and other message-passing languages, so things might get a bit tricky there...
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