Seems to me there are a couple of confusions/misunderstamdings here.

1. In my experience, rental cars companies will not ask for an IDP even if the country requires it. So, until you get stopped/are in an accident you will usually get away without one.

2. This never applies for EU countries licenses.

3. Technically the IDP is just a translation. It's not valid without your original DL.

4. AFAIK, the only European countries which technically require one are Italy, Spain and I believe Austria.

Surely a web search for these countries will yield the answer.

Specifically about Spain (just back), a recent Hertz confirmation reservation states:

"Warning Messages:
NON RESIDENT RENTERS REQ. INTERNATIONAL DRIVERS LICENSE"