This is a question that is frequently asked. Before or while being in Japan, we ask ourselves this question.
Before I go into what I think is the answer, let me explain the situation here. Japanese people do not depend much on English. They use their own language in everything, starting from conversations up to the Engineering courses they teach in universities. So, it is hard to find Japanese people who fluently speak English.
Does that mean I have to learn Japanese before I can enjoy living in Japan?
Notice the stress on the word have. Because this way, the answer to the question is NO. You dont have to learn Japanese so you can enjoy your time, but you better.
For me, I spend most of my time in my lab at the University of Tokyo. And eventhough this university is considered the legendary ultimate university in Japan, and Asia, only very few students can make conversations in English, surprisingly. So I am forced to speak Japanese. Yet, I do not have much time to spend learning Japanese and taking courses. So, with my intermediate Japanese skills, and their intermediate English skills, a conversation can be made.
Generally, I know cases of people who lived in Japan for over 5 years and still cannot read the hiragana. Also, people who lived for more than 7 years and still cannot speak a complete sentence. And I know people who lived and studies in Japan for years and are only able of introducing themselves.
That means, we dont have to learn Japanese to survive. But I say you wont be able to enjoy then. I like the language so much and I totally wish I had more time to study Japanese and specifically study more Kanji so I would be able to read. I cant though, while in the middle of the research.
As a summary, Japanese is not essential to live in Japan, while its still important and better to have.