Husbands, wives and partners (INF 4)
Copy/paste from the website:
What supporting documents should I include with my application?
You should include all the documents you can to show that you qualify for entry as a husband, wife, civil partner, fianc?, fianc?e, proposed civil partner, or unmarried or same-sex partner. If you do not, we may refuse your application.
As a guide, you should include the following:
Your original birth certificate.
Your original marriage certificate (if you are married) or your original civil partnership certificate (if you have registered a civil partnership).
Recent bank statements or savings books for your sponsor and evidence of your sponsor's employment in the UK, which could include payslips to show what financial support you have.
Evidence of your accommodation, such as a mortgage agreement or rental agreement, and evidence that you and your dependants can stay in this accommodation if it is rented or provided by your local authority.
Letters from you and your sponsor that are relevant to your application.
If you have been married or in a civil partnership before, one of the following original documents:
a divorce certificate
a final dissolution order, or
your or your sponsor's previous husband's, wife's or civil partner's death certificate
Evidence that your sponsor is settled in the UK. (This can be a copy of their passport or registration certificate that has been confirmed as a true copy, in other words 'certified').
I can't see anything there about translations - looks like they want the
original documents (which will be in your fiancee's language of course, i.e. Spanish).
Read through everything on this website.
General information (INF 1)
Then have a look at all the headings on the left hand side including biometric data.