Waiter does the usual thing and brings the receipt back for him to sign. They decide to tip in cash so he leaves the tip portion of the receipt blank and simply writes the total at the bottom and signs his name. The waiter takes the receipt and wishes them a good night.
Moments later, they are getting up to leave and the waiter runs over and stops them. Caps used to emphasize the fact that he was doing the "speaking in English really loudly makes foreigners able to understand everything you're saying" thing. Never mind that they're fluent and he's yelling in the middle of an upscale, busy restaurant.
"SIR. EXCUSE ME, SIR. *pulls out receipt* THIS PART OF THE RECEIPT *point point point* RIGHT HERE *point* THIS IS WHERE YOU WRITE IN THE TIP. SEE, YOU'RE SUPPOSED TO WRITE THE TIP IN RIGHT THERE *point* AND THEN YOU ADD THE TOTAL AND THE TIP AND YOU PUT THAT NUMBER ON THE BOTTOM LINE, NOT JUST THE TOTAL. THE TOTAL PLUS THE TIP. YOU HAVE TO PUT IN A DOLLAR AMOUNT ON THE TIP LINE IN ORDER TO TIP."
The visitor explained, in plain English, that his tip is sitting on the table right in front of them. The waiter seemed unphased and wished them another goodnight and left.