Mostly Java.... some scala (some teams might use it), and potentially other languages. It built its own api lightweight java framework around 2014-2015 (called Apollo), which is actually pretty decent/nice to work with. There was a lot of protobuf usage early on, but most teams started transitioning to plain json.

So, each team runs its own servers, for the features it is developing/maintaining, and most of the api goes throw a common gateway, etc.. etc... don't want to give out more than this.

Some of the simple front end web pages (i.e. marketing websites, analytics) might even be on php/python/ruby. Some teams wanted to use javascript (Node), but those are not the critical main app, as java is pretty much required to be used for anything that affects the main app.

Also, if you are doing machine learning, then whatever it gets the job done (usually Python).

btw, I left few years ago, so things might have changed.... I know at least one team that wanted to use GO for some of their services....