For Spain, according to spain.info:
If you are the citizen of an EU member state, Switzerland, Norway,
Iceland or Liechtenstein: you only require your valid driver’s
license. If you are from another country you will require an
International Driver’s License.
The IDP is just a translation of the license; the one I got had labels in French, Arabic and English; even though the actual license has labels in English as well. I was traveling to Malaysia and it turned out the IDP was unnecessary.
It is normally not a long process - these are offered by auto clubs. You have to supply your current license and a fee. Sometimes they may ask for another form of ID. I received mine in 15 minutes.
You may still be subject to additional rules and restrictions based on the local laws. For example, you may be entitled to drive a large vehicle but there may be an age restriction where you are visiting. Be mindful of these laws/variations as you are assuming complete liability when getting behind the wheel.
Given the above, from a practical standpoint I have had many friends (with non-EU, but digital licenses) that have rented cars and drive in Spain with out a IDP.
Since an IDP is so simple and relatively cheap to acquire (vs. a fine or worse in Spain), I would get it.