Is it only Python that can be used with prefect?

yes, you write flows in Python. You can orchestrate other processes from Prefect too, but the flow code must be in Python

So I can write a flow in Python and call out to .net? I assume at that point calling out will either be a invokation of a console app or a webapi? At which point don't I loose visibility of logging? Regarding. What's going on in the said console app? Anyone got any examples?

You may invoke such non Python process either as a subprocess, starting a Docker container or running a namespaced pod job on Kubernetes - for 1.0, check out the task library with Docker and Kubernetes tasks for that, there is also ShellTask for subprocess call For 2.0 there is currently only subprocess task installable with: pip install prefect_shell