The most common and most likely answer to this crossword clue is the 10 letter word SWEAR-WORD
I've seen this crossword clue in Financial Times
S
W
E
A
R
-
W
O
R
D
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 = swear-word.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.