alt Hacker NewsThe data sources often exist to build these things yourself. For your example there is Open Food Facts and USDA FoodData Central.
https://world.openfoodfacts.org/
Both would make a great foundation for this sort of app. OFF is crowdsourced and does include barcode information. I have no idea how robust the dataset is for your geography though. If I were to build something like this for personal use, I'd be looking at a PWA that can leverage the camera for barcode scanning. I'd work with the existing crowd sourced database as well as provide a mechanism for "manual" entry which should just be scanning a barcode and taking a picture of the nutrition information. I've personally built systems like this before and all of these things are well within the capability of most SOTA LLM to build out.
I am not going to even try until it becomes a literal weekend project with zero maintenance and perfect reliability. Maintenance is crucial - I would not want to become the maintainer of two dozen apps that are genuinely useful to me. At 10€ a month the economics are not there. Even researching if the codebar database has perfect coverage in Spain like MFP sounds like more time than I would like to invest. I have a thousand other projects I’d like to build.