Developers are always akin to learning new technologies and are on a lookout to employ cutting edge technologies on projects that they are working on.
The chief technologist would often guard against this to ensure that abrupt technology choices are not made merely to satiate the developer’s geeky hunger. In the monolith world, the job of guarding is easier because the technology stack itself will throw in limitations and make it hard for the developers to plug in other languages.
Microservices makes it very easy for developers to use different technologies for various services they are building. If the application now has several different languages, it can become a nightmare for the organizations to maintain the application.
Microservices does give you this flexibility, but it should be used carefully.