Au secours, toute mon équipe s'est mise à coder en rust

Something strange happened at Clever Cloud: everybody has started using rust, little by little. Many greenfield projects are using rust. That's not to say we've stopped using scala, java, or ruby (and even a bit of go and haskell). As a CTO, how do I keep complexity and language diversity in check? How do I choose which language to use for projects? Do I let others choose?