Just apply.......there are lots of information in the web and I am sure you can find it. There are a few universities in Japan that offer classes in English but subjects are limited. If not you will need to take at least one year full time Japanese class before you can really start your degree as all classes will be in Japanese. You need to read the website to know what is the qualification needed.  I did my post graduate in Japan but since its quite specialized its all in English.

Yup I have been working here for a few years even though my Japanese is not at the business level yet. Well the question is do you still want to spend 2-3 years of your time and money to do your masters? really depends what you want.

Like what sacwon mentioned,
Never ever retire in Japan, unless you married to a local. Then let's call Japan is your home. But not retirement after work from different country, confirm your savings drain faster because of the unwritten rules tax.
Thank god the artist agencies I used to work help me to settle all the tax for me.

Migrate to Big Cities like Tokyo or Osaka, Actually overall whole Japan while working with Corporate Company as Salaryman is not a good choice if you know hows the tax and retirement ponzi scheme works here.
Unless you know hows the market in Japan where you believe can earn decent money until you have enough savings without worrying paying tax for the elderly. Then different story.

If you have the cliche skills that available in Japan. For example my case taking Celebrity/Talent Photograph or Photograph for certain brand related with People product, then you have the chance to work under agencies representative, very competitive but you'll get certain job because the market here is over-saturated.