Sandwich is eaten in Japan and it is very popular and common food too. May be it is eaten more than in continental Europe.
Then, sandwich is eaten more as a take away, but it is normal to eat inside the restaurants and cafes. May be Europeans think that Japanese eat rice all the time but nearly half of Japanese eat bread in the morning too. Then, Japanese make sandwich for picnics and the variations of the sandwich is may be more than in Europe. Japanese try to put anything possible to make a new sandwiches. Not only ham and cheese, major ones are eggs with mayo(eggs are hard boiled and mashed), tuna with mayo(Japanese word for canned tuna is SEA CHICKEN), fruits(various fruits as like peach, pineapple, strawberry and kiwi and cut into small dices and mixed with whipped cream) and fried pork schnitzel. Actually the fried pork schnitzel is not like the one in Europe but it is Japanese one, TONKATSU. Then, if it is for sandwich, a word KATSU will be used.
According to the legend, Sandwich was created by Mr.Sandwich of a noble family and he liked to play card games - may be poker. His servant told him that it is time for a meal but he was so concentrated on the game, he asked the servant to put the food between the breads and he would eat using just one of his hands. I assume the servant put the ham between the two slices of bread. Nowadays, Japanese have adopted the word sandwich as SANDO and SANDO can be mean sandwich or interpose.
Japanese usually like to shorten the words, therefore mostly no one in Japan now says sandwich. Therefore if you are in Japan, please use the word SANDO instead of sandwich.
"Can I have a katsu sando?"