Fix front matter of zh-cn translation of scala 3 book

[bishabosha]

e.g. https://docs.scala-lang.org/zh-cn/overviews/scala3-book/why-scala-3.html

without the front matter, the pages all have the wrong url, and have no layout applied

Edit: they still need the language field in order to have the language dropdown

[bishabosha]

@benluo - just pointing to this issue if you still want to do that

[benluo]

I think it has been solved. I suggest to close the issue.

[bishabosha]

they are still wrong because they do not have the language field, so the language drop down does not appear

[benluo]

I add languages in all files in scala3-book. But they does not pass compile. How should I do?