Language school requirements

I'm interested in applying to a language school for an one year course. Beside the bank statement, is a certificate of employment also necessary? My financial sponsor has enough money to support me, but doesn't work at any company

From people that I have talked to, the primary thing that you will need is money. It can cost something like 16,000 dollars (around 1,750,000 yen) or more. Note- that would include housing/rent and other things. The majority or all of the money has to be paid up front. Arguably it would be a matter of if your "financial sponsor" will put up the required amount of money, up front. That's if you want them to help get you the student visa, and you are coming from overseas.

However, it is possible to apply to language schools in Japan, where the person already has a visa (spouse, long-term, etc...). Not sure if this is what you are talking about.

If you already have a visa and are in Japan, then the requirement can be monthly payments (after an initial sum). Depends on the school. Figuring out total costs can get complicated, because housing can be something the applicant can take care of this themselves. Such situations would be case by case.

The certificate of employment usually wouldn't come into it, unless the person is already in Japan and has a visa. Otherwise, they would usually be using the bank statement, to show the person can afford the school. Probably they would have your "financial sponsor" sign a contract guaranteeing payment or make a large up front payment. The bottom line for the Japanese language school, will be about the money. If you can make their payment demands, things will work themselves out.