Nutty or not, it happens. I was a Ruby dev and was asked to work on a Scala app. I could contribute to Ruby stuff quite nicely, and could hardly figure out how to compile the Scala app. Trade-offs...
I'm sure it happens - my point is that it isn't relevant to microservices. Microservices allow multiple technology stacks, if dont properly, but it doesn't force it on anyone. If your developers are pushing code using a different programming language with no oversight there's an organizational issue.
That said, it's awesome that microservices let you use different tech stacks to solve different problems.
Were you experienced with micro services before starting your latest thing? When starting a project, you're going to have constraints: money, time, available talent, management's blessing, etc. I'd guess those constraints are probably a driving factor in dictating how all of the project's needs are wired together.
If the technical founder was a Python/Flask/micro-service/Angular/MySQL dev, that's probably what they'd be using to knock out code to build an MVP. If the founder was a Microsoft-MVC/C#/Postgres/Ember/monolith dev, I'd be super surprised if the MVP was a Python/Flask/micro-service/Angular/MySQL app :)
That said, it's awesome that microservices let you use different tech stacks to solve different problems.