Here's why:
* It's not a proper noun: Sign language is a general term, like "English" or "French," referring to a broad category of languages.
* It's not a specific language: You would capitalize the names of specific sign languages, like "American Sign Language" (ASL) or "British Sign Language" (BSL).
Examples:
* Incorrect: She learned Sign Language in college.
* Correct: She learned sign language in college.
* Correct: She learned American Sign Language (ASL) in college.
Think of it like this: you wouldn't capitalize "languages" or "music," but you would capitalize "French" or "Jazz."