From what I recall from the info I have on the boat then if you are more than 6 miles offshore and doing more than 4knots then you can dump your tank (Spanish waters).
In Spain, holding tanks are required for spanish registered yachts.
Assuming you fly any other flag, your yacht has to comply with your national regulation.
... But I have never heard of the spanish authority to check non-spanish boat. They probably are not even allowed to, except in case of real suspicion of serious wrongdoing (drug traffic being the most frequent reason).
Don't worry!
I don't think you'll have a legal problem without a tank but would you really dump toilet waste when anchored off a bathing beach or in a clean Cala? We use ours all the time, pumping out at sea when appropriate. In any case, having been in the holding tank for hours or days, the first stage in breakdown of the sewage occurs and the solids break down making it much easier for the environment to dispose of it quickly - provided you don't use chemicals which actually preserve the sewage (ugh!)
What is unacceptable is turds floating onto a bathing beach.
Don’t know about France or Spain, but in three years of touring Belgium and Holland I’ve only seen three toilet pump-out stations, so if you do have one installed, make sure you can empty it yourself!
When we bought our boat in Holland I couldn’t find the pump-out skin fitting, when I asked, the broker said “Well you just go out into the river to empty it don’t you”?
In fact, we do have a holding tank but no other way of emptying it than dumping overboard. Just remember never to use the deck-wash pump whilst doing so!
“If you live with a sea-toilet, don’t buy the cheapest, buy the thinnest loo-paper you can”