The most common and most likely answer to this crossword clue is the 9 letter word PROFANITY
I've seen this crossword clue in Financial Times
P
R
O
F
A
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More clues leading to the result PROFANITY
Buddy explains!

The straight part
We know that "Bad language" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bad language = profanity.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PROFANITY mean?
PROFANITY noun- The quality of being profane; quality of irreverence, of treating sacred things with contempt.
- Obscene, lewd or abusive language.