What does ça mean in this case?
Comment ça se dit en français?
Comment = How
se dit = (they) say
en français = in French.
Isn't it? But what is 'ça'? Why is it needed?
We don't use 'ça' in every question with 'Comment', do we?
"ça" = "cela" = "ceci" - it mean "it" or "that" or "this" depending on context. In this case "ça" is the subject.
Literally: How that is said in French? (= How is that called in French?)
Even more literally: How that says itself in French?
As you can see, it is rather silly. You should have not separated 'ça' from 'se dit' because the full expression is 'ça se dit', which means something like 'it is called'. We can also say: 'on dit ça' instead of 'ça se dit'. 'Comment on dit ça en français ?' Translation: 'How does one say that in French?'
ça means ' it 'or ' that ' and is a pronoun , so in this sentence it should be 'How do they say it in French?' And if you want to know if we use ça in every question with ' Comment ' I found a article which might be helpful - http://www.fluentfrenchnow.com/how-to-use-the-awesome-pronoun-ca/