Scala (https://github.com/scala/scala) is completely developed on GitHub with Pull Requests requiring reviews being the only way commits can enter master. Issues are in a separate JIRA instance, although there is currently some experiments about migrating that to GitHub as well. A lot of other processes about the language, such as discussions about language features and their design, also are open on GitHub.

Its little brother Scala.js is entirely developed on GitHub too, complete with language proposals and issue tracking. https://github.com/scala-js/scala-js