To see your language percentage being used in your repository, simply click anywhere on the color bar…
and you’ll see the breakdown of languages detected inside.
If you want to see how GitHub is calculating these percentages, check out the github/linguist repository!
If you see the wrong language being reported for your repository, you can open an issue over at github/linguist or submit a pull request. See their contributing guidelines.
I was sitting next to a friend at a local conference listening to a How to use GitHub Pages presentation. She said she was having trouble following along because she had made a mistake somewhere. She pointed at her screen and said GitHub was showing her errors in her newly created Pages repository, because she only saw a solid red bar at the top. I explained to her that the bar represents the language being used, so in her case it was a pure HTML project. I told her that I had no idea the repo bar showed these percentages either until just a few months prior. I thought the bar was just a colored div since most of my own projects are just 1 language 
Sara Ford's Blog 