[armchairhacker]

Handle proposing a new language or framework the same as any other proposal: figure out the benefits and drawbacks, and decide if the former outweigh the latter.

Adding a new language or framework to an existing organization has big drawbacks: it requires developers to learn it, adds complexity overhead, and if the project using it integrates with any other projects, well, integration between most languages and frameworks isn't very good. If the organization is already using several languages and frameworks, presumably they cover a wide-range of use-cases, so they're less likely to encounter a new use-case that only a new language or framework solves.

But I'd strongly suggest not to implement a fixed "company will use N languages and M frameworks max", because it depends on the situation. It's similar to enforcing strict coding conventions (e.g. "variable names should be between N and M characters", "functions should be under N LOC", etc.). Sometimes there is a good reason to write unconventional code, and similarly, sometimes there is a good reason to add a new language, no matter how high the number of already-used languages is.