1. What’s vaguely appropriate. You’re probably not going to write C++ for browser frontend, and JavaScript is probably not a great choice for embedded. But there are a zillion equally valid options for web backends, for instance.

2. Ecosystem. I might choose a language for a game based on available frameworks. Or Python for ML based on available libraries. Or if you want to work with XMPP, Erlang is handy to know.

3. Familiarity. You’ll move faster and write better code with a language and ecosystem you and your team are already familiar with.

But often there are many valid options and you just have to choose one.