Depends on what you mean. Helm will control a lot. You can make the yaml file in any language. Also you can admin it from command line tools. So again any language but often zsh or bash.
On the platform consumer side (app infra description) - well schema'd yaml, potentially orchestrated by helm ("templates to hellish extremes") or kustomize ("no templates, this is the hill we will die on").
On the platform integration/hook side (app code doing specialised platform-specific integration stuff, extensions to k8s itself), golang is the lingua franca but bindings for many languages are around and good.
++, most controllers are written in go, but there's plenty of client libraries for other languages.
A common pattern you'll see though is skipping writing any sort of code and instead using a higher level dsl-ish configuration usually via yaml, using tools like Kyverno.
I'm seeing a lot of custom operators written in Rust nowadays. Obviously biased because I do a lot of rust myself so people I'm talking to also do rust.
If you’re talking about connecting to Kubernetes and create resources programmatically, Pulumi allows you to interface with it from all the languages they support(js, ts, go, c#, python) including wrapping up Helm charts and inject secrets( my personal favorite).
If you want to build your own Kubernetes Custom Resources and Controllers, Go lang works pretty well for that.
