My gut feeling is that microservices will lead to language unification within a team in the long run. Yes, initially, as you drop artificially imposed language restrictions, you will get a zoo of systems, but after a while (as the team experience grows) it will converge to a few or even a single language with DSL capabilities, it's just natural. In my experience, the driver for language heterogeneity is not languages themselves, it's craving for DSLs.