GitHub – Get Language Breakdown of All Repositories by User

Nowadays, GitHub supports archiving repositories.

Archiving repositories

You can archive a repository to make it read-only for all users and indicate that it's no longer actively maintained. You can also unarchive repositories that have been archived.
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1. On GitHub, navigate to the main page of the repository.

2. Under your repository name, click ⚙ Settings.

   

3. Under "Danger Zone", click Archive this repository or Unarchive this repository.

   

4. Read the warnings.

5. Type the name of the repository you want to archive or unarchive.

   

6. Click I understand the consequences, archive this repository.

Read more at GitHub help on Archiving a GitHub repository

I asked support@github.com; this is their reply:

We use Pygments (http://pygments.org/) to do syntax highlighting and determine which lexer to use based on each file's extension. So unfortunately, it doesn't look like there's a way to get C++ highlighting without renaming the files from *.c -> *.cpp and *.h -> *.hpp.

You can do this without losing git history of a file by using the git mv command.