I think it makes sense to have a preference for a single language in code bases when all of your developers only have one language in common and are not interested in learning any more languages in the future. That doesn't necessarily make it a golden rule.

However, in my work I've seen plenty of developers with all manner of interests and experiences align only on one or two languages, and if that's your company's talent pool, single language code bases seem like a good idea.

Of course this skips over all the usage of scripting languages (makefile/bash/Python/XML) which in my experience are seen as quirks of build tooling rather than part of the product.