Custom language in github repository

You could send PR to this repository (https://github.com/github/linguist). Just adding a language is not difficult, but GitHub staff accepts only languages that are used in many repositories. Let's read contribution document: https://github.com/github/linguist/blob/master/CONTRIBUTING.md

There is currently no way to do that.

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If your language was already used in hundred of repositories on github.com, you could add support for it in the Linguist open source project.

This feature has already been asked twice on the Linguist repository: #2627 and #2598. There are, however, some technical and legal concerns to address before this could be implemented (see #2627 in particular).