I've found this:
https://www.bancsabadell.com/cs/Satellite?blobcol=urldata&blobheader=application%2Fpdf&blobheadername1=Content-disposition&blobheadervalue1=attachment%3B+filename%3DMANUAL_TPV_VIRTUAL_ANG.pdf&blobkey=id&blobtable=MungoBlobs&blobwhere=8000012855489&bsb=R0JTX011bHRpbWVkaWFfRkEtNjAwMDAwNjc3NjIzMC0xMTgzMDE1NjQwMzU1&ssbinary=true
This
http://www.catalunyacaixa.com/docsdlv/Portal/Ficheros/Pdf/en/tpv_virtual_comercio_eng.pdf
And this
https://docs.spreedly.com/payment-gateways/redsys/#
All of them in english
We are deciding about WHMCS and Blesta, but the support of Redsys/Sermepa(it's the same, and include all of the major banks of Spain) is crucial
Hope it'll be helpful
Thank you.