Usually, yes. Spanish consulates and authorities generally ask for foreign birth certificates to be apostilled and accompanied by a sworn translation into Spanish, for example for children on a non-lucrative visa or for a nationality application. A few offices waive it for English documents, so check first.
The Consulate General of Spain in London, for instance, asks for the birth certificate of any child joining a parent on a non-lucrative residence visa, apostilled and with a sworn translation into Spanish. Its own consular Civil Registry, on the other hand, currently says documents in English, French, Italian or Portuguese don't need translating for its procedures. Same consulate, two answers, which is a very Spanish administrative experience (said fondly).
The rule of thumb: if the certificate is going to an office in Spain, a consulate's visa section, a notary or a court, expect to need a traducción jurada. If it's going to the consular Registro Civil in London, ask before you order. Tell us where it's going when you upload, and we'll flag anything that looks off.