I am just wondering, are you going alone or with someone or meeting someone there? If so, do they speak Japanese?
Why will you visit Japan ONLY if you are happy with your Japanese level by then?
For a short visit, you don't really need to speak Japanese unless your reason for going there is have conversations with Japanese people, otherwise all important things, signs, info for tourists are in English and most store, restaurant, hotel, airport, workers have someone who speak English well enough to help you. I will assume you will stay within a major populated area and not plan to go wandering off in the middle of nowhere.
I understand it would be more fun and convenient to be more fluent in Japanese, especially if you are going to be there alone and be intimidated by a different culture and language, but it is not a necessity just to visit Japan for 2 weeks. I went there for the first time, all ready with speaking basic and common tourist phrases and had my English/Japanese dictionary and translator ready to use, but I never had to use it, because it seemed whoever I needed to speak to for any important information, knew how to speak English.
If you were to stay there a month or more and really travel around, then yes, I would say it would be better to be more fluent in Japanese. If you are planning to go there and stay there as a guest in the home of people who only speak Japanese, then okay, I understand wanting to be more fluent in the language.
Japan has a great network of trains. You can easily travel from one city to another in a short amount of time. Planning train trips from places like Shinjiku, to Akihabra, to Yokohama to Kyoto to Mt Fuji, and so on is fairly easy and can visit many places within the 2 week visit.