I'll admit I haven't seen much of Objective C (apart from on OSX) - while C++ is pretty much available on anything and anywhere. However I'd also recommend having a look at C#. If you are not allergic to Microsoft products it's got a lot to recommend it:
But stay away from Entity Framework and Nuget. EF is slow, and Nuget....well, even Microsoft evangelists criticize it.
Cross-platform tools like Xamarin (which as an interesting ecosystem for cross-platform mobile apps)
If you do want to do cross-platform development, you should stay away from advanced C# features like code-contracts. In fact, if you really want to do that, then you should have at least one person developing on Linux, so you stay aware of all the little incompatibilities.