Our current Testnet is implemented Scala (P2P & Consensus) and Rust (Execution Engine). However, we are moving towards a Rust only implementation, which we are expecting to launch in Q3 as Testnet Delta.
We support contracts in Rust or AssemblyScript, allowing developers to write blockchain apps in any Web Assembly (WASM) compatible language. We also offer a full SDK with a VM runtime environment for Rust, complete with a test framework. Developers can use the IDE of their choice, and easily run their contracts in their Continuous Integration environment. This means over 95% of developers can write a blockchain app on CasperLabs network without any special training. This is a huge advantage.