•\x05when it comes to sth中的 it到底是什么成分?指的是什么呢?When it comes to German,I know nothing.

•\x05when it comes to sth中的 it到底是什么成分?指的是什么呢?When it comes to German,I know nothing.
•\x05when it comes to sth中的 it到底是什么成分?指的是什么呢?
When it comes to German,I know nothing.

•\x05when it comes to sth中的 it到底是什么成分?指的是什么呢?When it comes to German,I know nothing.
是真正主语.When it comes to German,I know nothing,这个句子可以改写成为：When we / you / people talk about German,I know nothing.
也就是说,when it comes to something=== when we / you talk about something,
when the topic is about something
看例句：When the topic is about travelling,where have you been in China?
=== When it (it== the topic) comes to travelling,where have you been in China?
所以,it===the topic.(主题).when it comes to sth的翻译是：在谈起 -- - -时,-- - - -.

it只是一个形式主语，没有实际的含义
when it comes to sth是一个固定搭配，意思是当提到。。。。的时候

固定句型，当然是主语，无实际含义

上句的意思大概是：说到德语，我什么都不懂！其中的it是形式主语，没有实际意义，是为了句子的结构而用在里面的。