A solo developer might well have the most reason for a polyglot architecture: they won't have time to write everything personally, but the best or most convenient libraries might be in different languages. Maybe the SaaS provider you want to use wrote libs for their complex API, but only in languages you hadn't chosen. Or whatever. As long as it's not something you'd need to call dozens of times per request, you can quickly spin up an microservice using almost any language to front-end that must-have library.