C and C++ are probably the only viable languages for true cross-platform development.

The Salami Method

by Adi Shavit

From the article:

The Salami Method finely distinguishes between the different aspects and layers required for exposing platform-independent C++ on different “specific” platforms. At its extreme it strives to create a single, thin, transparent layer for each such aspect so that each layer is more easily built, tested, debugged, managed and maintained.