Hello, I had been living in Japan for study and work for 13 years
I met people quit their job and study in Japan.
Some people get a master degree , some people go to college to get diploma and some people enrol into language school.

Your case is to study Japanese Language. To the best of my knowledge, only private institution offer Japanese Language Course.
For private institution, the school fee can be around 0.7 to 1 million yen( approx. RM 26k to RM 38K).

Beside the school fee, you can find part time job to support your living expenses. Many part time jobs available in Tokyo.
Wages is around 800 to 1000 yen per hour( approx. RM30 to RM38)

Compared to other advanced country, Japan is much easier to get part time job.

Hope this info helps.

EuroDesign

Oh ya, thats right. It is always better to have some backup plan.
Good luck