Sounds like you have experience working in a development environment, so the transition should not be difficult (coding is part of it. Other things such as testing,refactoring,good design take up more importance as well). Small projects would be great just to get yourself familiar with the syntax of those languages, but if you are comfortable with the .NET, a transition to Python/JavaScript should be relatively easy.
I would say this though. You can leverage .NET with open source via .NET Core.