There seems to be some very strange rules about foul language, some will be caused by typos and some because of different languages with words meaning different things. However, this is not always the case, and careful re-typing still produces the foul language message when nothing even remotely foul has been typed.
One example was a Christmas message about a famous Charles Dickons character called Scrooge - which was fine, but his first name "Ebene....." cannot type it in full as it comes up as foul language.
Then on a quest location - Ruins - you have to chat to various characters and the word "Bast8rd" is used by you several times - that word is very definitely FOUL language!