There wasn't an option in the developer console for platforms like WP8 to choose from, but they added a "other platform" option recently.
WP8 also supports native code and the API is just C++.
But I guess you are referring to this: Set up a platform configuration for your platform (iOS or Android).
It would be no problem to use the API on a WP8 for sending/retrieving the data. The only red herring is attaching the view for WP8. Maybe they add it in the future.
They also released a .NET API especially for WP8:
I did not try it myself, but again, the API is just C++ and the new API google provided is .NET.
There is also a REST version: https://developers.google.com/games/services/web/gettingstarted
You can always use that.
The Android version is implemented in Java, the iOS one in Obj-C and the C++ version in pure C++. The latter will work on any device/platform able to compile C++ code. The "problem" is just the platform configuration, to init the services.
All the APIs are just sending web requests/responses anyway(besides the "fancy" service icon overlays).
Besides all of this, it's important that you read the fine print, which services are allowed to be used within specific environments. E.g. IAP. On some platforms you have to use the API of the platform owner(Apple, google(but it seems they opened it up), and on others you are free to use third party libs(Amazon).
It's not always a matter of an API/language, but platform specific rules you have to obey to. That's the price you have to pay for using the platform owners services for "free", well, 30-40% of your revenues