This topic is already a bit old, but for those who look for an easy way to disable monster chat:
<?xml version="1.0" encoding="euc-kr" ?>
<enemy_monster_talk_table>
</enemy_monster_talk_table>
Copy those lines, create a new text file, copy paste the lines in there, and save the file in your data folder, name it "monstertalktable.xml". No more monster talk!
If you want to have monster talk back, just remove or rename the file.
Let me just clarify that not ALL client side modifications are allowed. Modifications which give you an unfair advantage over other players are obviously NOT allowed. Removing monster chat, on the other hand, does not give you an advantage over other players, so I don't mind if you disable it.