In the yesterday match we have drawn the "attack on San Juan" event, changing the nationality into English.
After a few rounds the "Lead the Way" event came out, but in this case may be more accurate exchanging the requisite and the ship with the one of the new owners?
I guess you could house rule that with out any issue. If you agree on it from the start.
Or you could just see it as continued Spanish influence in San Juan. Another nation may have conquered it, but there are still influential Spanish minded people there to give you the mission as it is written.
Edit: Though you do get an extra thing to think about, if a player takes the mission while San Juan is Spanish and there then the attack comes after before the mission is completed. Then you need to add/remember for the the San Juan attack event, to check if anyone has this mission - and if so place a marker to track, that the ship should be from Spain. Or just always place a marker, if San Juan is not Spanish at the time the mission is taken. Even simpler just to go with the card text as it is.