Valiant effort at old-school engineering applied to a niche problem. (Iceland has a population of only around 400,000 people!) As much as I love the geekery of this stuff though, isn't it already a better ROI to get an LLM to generate the strings you need? It has its own other problems (not claiming it'll be perfect) but for something so language related, it makes a lot of sense. Would also work for other languages that have the same problem with declension of proper nouns like Russian or Finnish.

The article describes that a government body is using this library to generate indictments. In that situation, you do not want something that is mostly usually correct. Indeed, they asked the author for a strict version that does not try to guess the declension of unknown names based on their suffix, presumably so that they can just not decline them, which is better than picking the wrong declension 0.05% of the time.