On Github, some projects are tagged as being of one language or another. How do I add one of these tags. (Objective C, PHP etc.)
A repository can only be "tagged" with a single language in the search results. That language is the first from language statistics. Language statistics are computed by the open source project Linguist according to the total size of files for each language detected.
You can override language statistics using Linguist overrides. In particular, you can choose to ignore some files from statistics:
directory/to/ignore/* linguist-vendored
*.somextensiontoignore linguist-vendored
or override the detected language:
*.js linguist-language=Java