My DS has lentil allergy and this is the first time I've ever come across anyone else with the same problem, other than in medical reports on the net! He broke out in hives the first time he had split peas, and the same with lentils of any variety (including poppadoms, which are made with lentil flour -- something I was unaware of until after he had eaten them and broken out!) and garden peas. We always carry antihistamine with us.
The allergy clinic told us that there is no way to say if an apparent first exposure to an allergen is actually the 'first' exposure, as many foods could be contaminated with the allergenic food, etc. For example, DS developed severe swelling on one side of his face after hading around a bowl of pistachios to adults -- he didn't eat them, but he must have got some of the oil on his fingers and then touched his face. As far as we were concerned, this was his first-ever exposure to pistachios.
He can now (8 yo) eat peas with no problem, but I kept him off them for several years. The spilt pea and lentil reactions were so severe that we never have them. The allergy clinic couldn't test for lentil allergy as they don't carry specific tests unless they are quite common, but I need to test him by putting a small dab of lentils in olive oil on his back, then if no reaction do the same with a small dab near his mouth, then if no reaction a tiny pin-prick amount in his mouth, gradually increasing the amount if no reaction. Of course this process is spread over a number of sessions.
Because of his allergy to both lentils and pistachios, the allergy clinic told us to avoid all nuts and that he is at 'increased risk' of peanut allergy. They also told us to treat soy as a nut and to avoid sesame and other items such as sunflower seeds, but he was already having them with no problems so they said that was okay. He is now fine with sesame and soy.
Kewcumber, lentil allergy may be more prevalent in Spain simply because they might eat more lentils there rather than the fact that they are Spanish per se -- I don't know enough about Spanish diets to say for sure though.
And as Tatt says 'lentil allergy doesn't automatically mean nut allergy', as it really would depend on what enzyme or whatever within the food is causing the reaction.
Sunflower, I would suggest staying well clear of lentils for a couple of years or so and talk to the hospital clinic about the testing method that I mention above, with a view to doing it when your LO is say 3 yo. Good luck.