Possible Clues:

• Tongue of thief a king received ...
• Lingo (GATOR anagram)
• Ms Fonteyn shunning Monsieur's vulgar tongue
• Insider speaking
• Specific speech of king in classical garb from the east
• Slang used by a particular group
• Particular language to investigate Monet’s work, as it were?
• Try penetrating cunning jargon
• A king acquired the language of a particular class
• Lingo of a particular group