Getting a British passport for a dual nationality child

[Superplonk]

For the last nine years my son has been unknown to the British authorities as the Spanish nationality he inherited from his mother has sufficed. During all that time I've fended off my own mother's demands that he get a British passport "in case anything happens." Until now I've told her that it was hard enough keeping up with the endless cycle of Spanish document renewals. With talk of a referendum in the air I've now decided to take some action.
The gov.uk website is fairly clear about the procedure, even stating that "unknowns" like my son are welcome to apply. Filling out the webform to get the printable application was quite straightforward and payment was accepted.

I have two questions about the documents that need to be returned with the application.
Firstly, in Spanish terms, what is the equivalent of a birth certificate? Is it the libro de familia, or something else that needs to be obtained from ...juzgados?
Secondly, the form states:

Documents that aren't in English must be fully translated by a professional translator.

Does this mean apostilled, or just done to a high standard by someone with a letterhead?

Thanks.