My first question would be: at what time of day is the wedding? Next...how old you and your friends are? Will you be traveling from the States specifically to the wedding?
Thirty years ago I was invited to a wedding in Madrid. My friends and I were living with the family directly involved...that's why the invite. The religious service was, for me, a hoot. Not as formal as here in the States. Everyone just ran into the church and the priest preformed the service, one,two, three. There was no "official" procession nor music. Nor did the bridesmaids wear the same outfit. They were, however, impeccably dressed. The reception, which was held in a 4/5 star hotel, was, from an American perspective, semi-formal: suits for the gentlemen and cocktail dresses for the women.
I would suggest that you ask if the function is "black tie (un smoking)", formal or semi-formal. August, moreover, is a brutally hot month. Even if it is formal, I'm sure you can find something weather appropriate.
Sheryl