Is $100 a day enough for Japan?

To be honest, $100 a day is a very limited budget for a trip to Japan. Only suitable for Backpacker Free travelers with apartments stay.

Travel Maps of Japan's Attractions，Distance between cities and required driving time.

Eating out in Japan is with widely price range from several dollars to hundred dollars. It's up to you to make it expensive or cheap.

If you come from a non-English-speaking country, the tour guide service in your language must be booked in advance with sufficient time

A full private tour model (car, driver & tour guide) is the best way to experience Japan with a professional guide! But the rate is expensive, especially for small size group. It is recommended to the people who are not budget or big group over 10 persons.

How can one in Japan obtain a National Guide Interpreter qualification certificate?

Sure, if you would like. With highly developed transport, extremely safe, and easy communicate, you are free and easy to do it on your own.

In the National Interpreter Guide Examination in Japan, ten foreign languages are offered, including English, French, Spanish, German, Chinese, Italian, Portuguese, Russian, Korean, and Thai

Japan offers a completely different landscape throughout the year. The best month to visit Japan depends on what kind of landscapes you're looking to experience:

If you plan for a trip to Japan with 5000 USD, you can expect at least a week trip to visit Japan (not luxury style).