Help: my company is firing me without due process

I'm a seishain at a small company in Tokyo. It's a Kabushiki kaisha, but the CEO, and most of the staff, are foreigners. I have been working there for almost 6 years.

The company is struggling financially and decided to restructure. Today, I have been invited in a meeting and told that I would be terminated. Later, they sent me an email (in English) to inform me that my contract will ends at the end of this month.

The people who wrote that email are not Japanese, and not familiar with labor laws in Japan (we already had problems in the past regarding vacations calculations and other stuff).

The company employs one Japanese contractor, few day a month, to take care of payrolls, contracts and legal things. But it doesn't look like they have been included in this termination process.

From my understanding of labor laws in Japan, to fire a seishain cannot be just: "send an email and give 30 days notice". Is it?

Can someone explain what the normal process should be? Should I receive formal documents? Something to sign? Can they even fire me like that? What about my remaining vacations? (I still have 26 days of 有休休暇残)