You've done an excellent job of summarizing why Google does what it does: extremely few languages, and all code must pass readability review (or be written by an author with readability for the language(s) used) before it can be committed.

Though reading code isn't just it, language interop everyone knows how to use is important too. While all the fancy JVM languages have Java interop, it's still a huge pain for every old Java developer who wants to call your Clojure library. They'll end up learning how to do it separately for every language, every time, multiplied by the number of developers who aren't fluent in these languages (most).