GitHub uses linguist library to detect the languages used in a repository. It ignores library codes from counting towards the language percentage. So if you have files that are not part of the main code which are not in the vender specific path and you wanted to ignore them from the statistics, you can tell GitHub to not count them by specifying it in .gitattributes file. Say I have a public folder which contains documentation or support files which I wanted to ignore, then the .gitattributes file will look like below.

public/js/libs/* linguist-vendored
public/* linguist-documentation

You can add linguist-vendored or linguist-documentation to ignore the files from counting. You can see the difference in the language statistics bar of your repository.