Clemens,
Just in case translation is causing confusion: you say that it is not uncommon to see "loafers" worn with dinner jackets in Spain--in Marbella that may be true, but it may be that you are seeing something else. Traditionally, there are two kinds of black slip-on shoes (no laces) that are correct with dinner jackets: the evening pump, usually in shiny patent leather, but sometimes in polished calf, with a grosgrain bow. The opening of the pump comes very far toward the toes:
The other kind of traditional slip-on for dinner dress is the Albert slipper, again in patent or calf, with or without the grosgrain bow (although the calf version without bow is dangerously close to a house slipper):
Are these what you have seen? If so, these are correct anywhere including the UK (the last example is from John Lobb, St. James), but NOT with anything other than white or black tie (or a dressing gown or smoking jacket at home).
If you have seen moccasins or Gucci-style loafers with dinner jackets, perhaps on the Mediterranean coasts, this would be an example of a regional informality (as Costi says, very degagé) that would still not be correct for most black-tie occasions in the northeastern U.S. or major cities in the U.K.
So I believe Costi has advised you well: if you are going to get true "loafers" (by definition a casual type of shoe) for your first bespoke investment, you might do well to get a style and color that will be versatile, so you can wear them more frequently than you will be likely to wear your dinner jacket--this way you will be spending the money where it can be the foundation of an enduring bespoke shoe wardrobe. You can get evening pumps, or dress balmoral oxfords (patent or lightweight calf):
in a good ready-to-wear model (assuming your feet are not difficult to fit) to wear with your dinner suit now, and perhaps replace them with bespoke when your bespoke shoe wardrobe is larger.
Or: you could get the bespoke evening shoes now as a pure indulgence, but you would probably be disappointed if you thought you could wear them anywhere else (unless you chose calf oxfords, and then you might wear them with sober dark suits).
But you will also probably be disappointed before long if you buy bespoke black loafers (such as moccasins):
and begin to feel out of place wearing them at black-tie events.
Good luck!