In a GitHub repository you can see “language statistics”, which displays the percentage of the project that’s written in a language. It doesn’t, however, display ho
You can clone just the latest commit using git clone --depth 1 and then perform your own analysis using Linguist, the same software Github uses. That's the only way I know you're going to get lines of code.
Another option is to use the API to list the languages the project uses. It doesn't give them in lines but in bytes. For example...
$ curl https://api.github.com/repos/evalEmpire/perl5i/languages
{
"Perl": 274835
}
Though take that with a grain of salt, that project includes YAML and JSON which the web site acknowledges but the API does not.
Finally, you can use code search to ask which files match a given language. This example asks which files in perl5i are Perl. https://api.github.com/search/code?q=language:perl+repo:evalEmpire/perl5i. It will not give you lines, and you have to ask for the file size separately using the returned url for each file.