Hey! Sorry i couldn't answer straight away. I was doing some research on this question. So here is Japans route!
First of all, be sure to select female gender. If you did already, then restart game anyway. Write your name and select gender once again. Then visit art club and choose "Anime" as an answer there. Since if you don't do it, Japan's route doesn't always work... I have no idea why.
Join art club.
When you make a poster with Japan, select these answers:
"Red-pink"
"Blue"
"Dramatic"
During math project, join Japan, Germany and Austria. Then choose "I agree with Austria"
Draw something during class whener you have a chance (and if it affects Japan, not Italy). When Japan asks what you are drawing, answer "Anime" and let her look.
During break, in the club room, when Japan asks you what kind of games you like, answer "Visual novels"
If you did everything correctly until this moment, Japan will ask "You also like drawing anime, right?". Anwer "Yeah, why?". Then she will ask to collaborate. You should answer "Of course!"
After that just wait until the classes end (choose to draw something during class once again until then).
After class, select "You're finally here! Let's go work on the dating sim!"
Say "Great idea." when Japan proposes to go to the cafe.
In cafe choose "Some boba, please!". After that choose "I hope we can go on adventures like that together.". Then agree to meet up with her after school.
On the next day, during break, you can go to club to meet with Japan. But there won't be any choices there. THOUGH you can give gift there.
After some time, during break in club, Japan will ask to help her with code. Answer "Sure!"
At Japan's house, agree to go out with her this weekend. (You can give gift here)
On weekends you will get a text from Japan. Answer "yes!!!"
When Japan asks what would you like to drink, ask for tea. Then say "You're so fast!"
After that answer "I love a slice of anime!"
After playing the game, answer "I did!". As for the favorite part, choose "I loved everything!".
After Japan tells about never being in relationship, say "I would never make fun of you for that."
When you have an otion to write a love letter, choose to talk to Japan. Then select these answers:
"Cute cakes"
"Peach is pretty good"
When Japan asks to meet in front of the school, choose "Alright..."
That's it! Congrats! If you didn't get the ending, try to restart game once again. Or always fall asleep during class. The reason may be in you being low on energy or something. There is also a second route. At least that's what the list of routes says. There is also a picture of it in game files. I've been trying to find it these past 2 days, but I didn't succeed... I really am sorry :(. All I could get is new dialogs. And even if I had good relationships with Japan there, game still didn't give me a chance to date her anyway. Maybe I'll look into this question further and find something more... I'll be sure to let you know if so!