Question regarding birth certificate.

So, long time lurker. But figured I’ll ask this here because can’t really find the answer.

Having a baby soon.
I am married with Japanese. We do not share same last name.

And from what I understand after reading here. On the babies birth certificate you can have both last names?

So when I do that. Is it in katakana or romaji or?
Like, what do they want from me?

Do I need to bring some papers or is my gaijin card enough?

Thank you

by Deep_Impress844