I feel like a year ago JavaScript and Python were the best languages for coding agents to use because of their heavier presence in the training data, but I'm not sure that's true any more now.

The latest frontier models are competent at Rust and Swift and all manner of other less widely used languages.

The more important factor is how good they language's compiler is at kicking out actionable error messages, since one-shot code generation isn't as important once you have a coding agent loop.