I second this request. Been wondering more about this process ever since I noticed, sometime in January, that Rust (a nascent language with very little use yet) was ranked in the low-20s for language popularity. The problem likely had to do with how the Rust devs registered their language with Github's Linguist[1] project; Rust's units of compilation ("crates") have the .rc extension, and Github must have taken this to mean that every project with a .rc file (notably many Android-specific projects) contained Rust code. The Rust devs have since revoked that association from Linguist[2], but since Github takes a while to reevaluate a project's language breakdown, the language's popularity ranking on Github is still artificially inflated (sitting at #31 at the moment).

[1] https://github.com/github/linguist

[2] https://github.com/mozilla/rust/issues/1512

My guess is the latter.

It's the number of occurrences of a language in all public repositories. Most web applications on GitHub have some javascript, for instance.