I would download the MyFitnessPal app.
I don’t know if it’s made to specifically work in Japan, but so long as you can get an internet connection (which should be easy there), then you should be able to still access the Western database Restaurantt food is always tricky. Foreign food even moreso.
MyFitnessPal let’s you search for various foods, change the serving size, see nutritional info including carbs/fiber, and record it if you want to track it. Plus many other features. I like the ability to enter my own half-carb recipes and let it figure out the math. I don’t know how well it will work in Japan, but I know in the US a lot of restaurants will upload their menus and nutritional info to the database, so it takes the guess work out of eating out.
For instance, this picture shows the nutritional info for one piece of tempura shrimp at Benihana. A good traditional version in Japan may vary from from the American counterpart, but it’s a good starting point. If you scroll down the app farther, it shows that the single piece contains 7.8g of carb and no fiber.
The hardest part is actually learning measures. You need to actually learn what a cup of noodles or rice looks like. Or equivalencies of weight to size. There are lots of guides online in the dieting community for portioning. I.e two dice sized cubes of cheese equaling one ounce or a palm sized piece of meat is three ounces, a tightly clenched female first is approximately one cup (no big man hands for that one), etc… Learning to eyeball the size/weight of food is seriously the key to success with this approach.
I’m pretty jealous of your travels myself. I love Japanese culture and food! Good luck