I’d expect at least a teaspoon of sugar on the fish along with the soy sauce and table salt. Every Japanese recipe I’ve seen uses white sugar for some reason. Maybe you could tell the waiter you’re diabetic and to hold the sugar. I wonder if anywhere serves juuwari-soba… Chinese restaurants will be even worse, but I wonder if Korean would be okay. Obviously requires tolerance to chilli, but there are side-dishes like kimchi and other fermented veg and crab or tofu soups as main. Maybe a moderate amount of yakiniku if meat doesn’t trouble you? I’ve never seen any Korean recipes, so I’m not sure what they’re based on, but the dishes I’ve tried didn’t seem at all sweet or suspicious in that regard. Otherwise, a good Indian place should have something (again chilli will be added) you can eat. A chickpea masala or some sort of dhal, for example. You’d want to test things like tofu, lentils, tomato etc in advance though.