Dear Dr. Boli: How should I excuse myself when I’m at dinner with good company in a decent restaurant, and I need to, you know, excuse myself? —Sincerely, A Gentleman Who Will Be Right Back.
Dear Sir: The principles of etiquette imply, if they do not state outright, that you should be bound by what the restaurant calls its facilities.
Restroom: “I’m going to go take a rest.”
Bathroom: “I’m going to go take a bath.”
Washroom: “I’m going to go wash up.”
Water closet: “I’m going to go put some water in storage.”
Comfort station: “I’m going to go make myself comfortable.”
Public convenience: “I’m going to be convenient.”
Men’s room: “I’m going to be a man.”
Facilities: “I’m going to facilitate.”
Powder room: “I’m going to take a powder.”
You see the principle, and can doubtless adapt it to the local vocabulary. Alternatively, you could just stand up and say, “Excuse me,” and etiquette would be perfectly satisfied with you.