Well Jagex Wishes That Every Player Can Enjoy Playing Runescape Without Offending Themselfs With "Offensive" Language. Rule One Directly From Runescape.com:
You must not use any language that could be considered offensive, racist, obscene or otherwise inappropriate. This includes swearing.
If you suffer serious abuse such as receiving racist remarks, harassment or obscene and vulgar language, please report them to us using the in-game Report Abuse button which can be found in the chat interface of all our multiplayer games
1. Why do we have this rule?
We have this rule to maintain a pleasant environment in our games. For many people it's not much fun playing if there is someone else being rude or offensive, so please consider others when chatting. Also please remember that not everyone playing is the same age or from the same background, so may be more offended or upset than you expect.