Tech zoo sometimes considered to be an anti-pattern in microservices. By introducing a different language into your organization, you decrease the mobility of developers between code bases and dilute technical knowledge.
Everybody shouldn't pull in their favorite stack just for fun, but it seems valuable to have the option of trying out new things that could turn out to be a better way forward for the org
Rewriting a module from one tech stack to another is not much harder when that module was a part of a monolith and not a separate service, except that you haven't paid the upfront cost of bootstrapping a new service, putting in RPC calls, etc. And in any case, starting a project as microservices is already a bad practice due to a number of reasons, the most important for me is that it's hard to change module boundaries, which you will most likely get wrong in a new project.