particles - How does the の work in 「日本人の知らない日本語」?

I've read that 日本人の知らない日本語 translates to: "Japanese (language) that Japanese (people) don't know". But I don't understand how or what the の does in that sentence. If I'm not mistaken 知らない日本語 could mean "Japanese language that (x) don't know" or "even unknown Japanese". But I don't get how the 日本人の fits into the translation.

Answer

In your example, 日本人の知らない is a relative clause, equivalent in meaning to 日本人が知らない. This clause as a whole modifies 日本語, so it means the Japanese that Japanese people don't know.

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In relative clauses, the subject particle が can be replaced with の:

1. ジョンが買った本


2. ジョンの買った本

The book John bought

This is true in double-subject constructions as well:

1. ジョンが背が高い理由



2. ジョンが背の高い理由


3. ジョンの背が高い理由


4. ジョンの背の高い理由

The reason John is tall

But you can't replace が with の if there's a direct object marked with を:

1. ジョンが本を買った店



2. *ジョンの本を買った店 (ungrammatical)

The store where John bought the book