4 languages doesn't really seem that many given how many engineers there are at google...

Yes - my only attempt was in the early 1980s when I started to develop a language implemented on top of interpreted BASIC. It was called MAL (My Attempt at a Language). I wish I still had it - I have no recollection what my design goals were, if any! The only thing I have left is the name, and I'm quite sure that was the best part about it anyway.

those are just the public ones

I can think of a few others…

Sawzall, a language focused around processing logs. Rob Pike led on this but use has pretty much all been replaced by Go. https://en.m.wikipedia.org/wiki/Sawzall_(programming_languag...

Dex, a language focused around array processing from the team behind the Jax machine learning library. Early stage research project. https://github.com/google-research/dex-lang

Rune, a language focused on security, early stage research project. https://github.com/google/rune

Wuffs, a language focused on writing safe file format handlers (parsing, encoding, decoding) https://github.com/google/wuffs

Cue and skylark (now starlark), too.

Also other internal languages: borgmon, borgcfg. Don't think they use borgmon anymore though.

Initial version of AngularJS by Miško could almost be considered it's own (compile to JS) language.

Inventing a language is a rite of passage for every engineer. Domain specific problems are best appreciated in the constraints (and flexability) of a domain specific language.

The internal GCL language, as well as its failed replacements. There's a nice introduction in this paper: https://pure.tue.nl/ws/portalfiles/portal/46927079/638953-1.... Back when I worked there, I actually quite liked the syntax of the language, though the semantics of the language was quite a dumpster fire.

Another internal language for querying Monarch. See section 5.1 of http://www.vldb.org/pvldb/vol13/p3181-adams.pdf

To be fair, it is a rite of passage for anyone doing a proper software engineering degree anyway.

Hence why there should be a very good reason to start an ecosystem from scratch, given how many languages get invented per year across the world universities.

Rune - posted very recently -

https://github.com/google/rune

https://news.ycombinator.com/item?id=33761193