A variation of this is to explicitly take a job at a company to solve a problem with one language / area, with the understanding that you will do different work

A variation of this is to explicitly take a job at a company to solve a problem with one language / area, with the understanding that you will do different work once that's complete. I did this with a consultancy a few years ago. I did some Symfony work for a client they needed help with, but there was an (honoured) agreement that after that I would never have to do PHP work ever again. After that I moved into Python & Golang work.