The Japanese language
Japanese is a very hard language; even for the Japanese.
To give you an idea of how hard it is to learn: it consists of 3 different kinds of scripts: Hiragana, Katakana and Kanji. Kanji has over 3000 characters with their own specific meaning (Japanese are supposed to memorize the 1200 most important ones); they have a different numbering system depending on the object you are counting. There are politeness forms depending on the position person you are speaking to. It’s just very hard.
There are many resources to learn Japanese online but for a short trip it makes more sense to memorize some useful words than to actually try and learn Japanese.
English in Japan
If you’ve never been to Japan you might think Japanese’s English is not too bad. Once you’ve been you’ll know that almost no one speaks English and almost no one is exposed to English culture. Bands that are world famous in the West (think Foo Fighters): a Japanese person wouldn’t know them. Then again, we don’t know the Japanese bands.