In some cases, this is unfortunately true. If you decide to take the c++ route, you may have trouble getting a game made for the Mac, as it does not have very good c++ support (it recommends objective C with Cocoa). If you choose Java, you will have the best multi-platform support, but the language is always changing constantly, and it is very slow compared to c++.
Huh? My engine is in C++ and my primary development platform is OS X. You need objective-C to interface with the Cocoa frameworks but everything else can be written in C++. There is even the so-called "Objective-C++" (.mm) that makes it easy to write C++ wrappers to the Objective-C stuff.