You don't need to know Japanese to solve the last puzzle: just look at the last message. One word is repeated several time and, every time, a part of it is written in our alphabet. So you can reconstruct the entire word just by looking at the message.
A few days ago, we have released an updated version with a hint system. Hopefully, it will prevent people for being stuck.
Thanks for playing the game! We're glad you liked it anyway.