请教英语问题

Question

My father has a large collection of books, many of them are written in foreign languages, which I have no knowledge of. 请问哪里错了？给我理由，谢谢！
答案是把them改为which,可是我不明白为什么

Answer 1

原来的句子如果不改的话就有两套主谓宾了（正确的两个句子的写法）：
My father has a large collection of books． Many of the books are written in ．．．．．．

如果把them改为which，就是一个非限制性的定语从句，从句子结构来看只有一套主谓宾，也没问题．

My father has a large collection of books，many of which are written in ．．．．．．

Answer 2

这是非限制性定语从句
两个分句中必定要用连词隔开，若不改which，要加and

Answer 3

一千个伤心得理由

Answer 4

在第一个分句后加and

Answer 5

WHICH改WHAT吧，不确定