You can get someone to go to the display window with you (if the restaurant has one) and point at what you want.

We'll demonstrate the Japanese "come here" gesture in class.

Or if the menu has pictures, you can point at that. Or if you can read a bit and guess a bit, you can point at the "picture-free" menu to order what you want.

Or you can pick out something likely-looking nearby and ask for more of the same.

But pointing at the guy in the next seat is rude. We'll show you how to indicate someone else without being rude.

Of course, you could work REALLY REALLY hard on your Japanese before you go so you can just ask for what you want.

Do be careful of your pronunciation. As one of our students learned, "mizu" (water) sounds a lot like "mitsu" (three) if you don't pronounce it clearly.
WATER is cheap. Large pizza, THREE isn't.

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