The latter. But you should realize Google is pretty conservative internally in the main repository because of the amount of code/tooling/etc. While we let people experiment in the main repository, they are controlled experiments. Even if we decided, tomorrow, to move the main repositories away from our current set of programming languages, that would be a fairly long term path, not a short one.
Outside of the main repository, or on google open source projects, or etc, people can generally do what they want, they just don't get the benefits of the above if they aren't using a language we already support very well.
Google's "big" client apps (Inbox / Gmail / Calendar) are a mix of Java converted to JS (GWT) and native JS with the Closure library/compiler. I think, anyway.
It's FUD aimed at a particular language, though. If you know anyone inside Google you probably know how regularly people "experiment" without pushback; all of the internal Dart projects, for example.
Outside of the main repository, or on google open source projects, or etc, people can generally do what they want, they just don't get the benefits of the above if they aren't using a language we already support very well.