Just to let you know.. Only one party needs to be catholic for you to have a catholic wedding in Spain
.. although church rules still apply, in that neither must have been married before..
Also within the Catholic church there are 2 type of ceremonies you can have and both are legal .. a full Nuptial ceremony (with holy communion) or an non Nuptial ceremony (without holy communion)
When a couple is of mixed religion and are unsure what type of ceremony to have.. the non Nuptial ceremony is always a good option because the ceremony is very much like that of a blessing ceremony, but is legal..
For non resident foreigners wanting to get married in a civil wedding in Spain.. Unfortunately then one party needs to be a resident / registered on the padron to have a legal civil wedding here.
Hope that helps..