I mean, it's an interesting problem for Icelandic sites, but because he's explaining the basic concepts of how declensions work, it seems like he's aiming this at non-Icelandic developers. If they were to use this, no doubt it'll end up butchering names in some other language and lead to all manner of hard to track down bugs.

For example, if an English person called Arthur uses the site in Icelandic, I'm not sure they'd expect their name to be changed to presumably "Arth", "Arthi" or "Arthar" even if they were a keen learner of Icelandic. Their name is their name. So, as well as storing someone's name, you also have to ask them what language their name is, or guess and get it wrong. At that point, you might as well just ask them for all the different forms for the name as well, and then you don't have to worry about whether their name is on an approved list or not.

And if the website isn't localised into Icelandic, I've also got to wonder if Icelandic visitors would have an expectation of Icelandic grammar rules being applied to English (or whatever) text. Most Icelandic people I've spoken to before have an excellent command of English anyway, and I'm sure they'd understand why their name isn't changing form in English.

Not sure how it is nowadays, but Iceland used to force anyone immigrating to officialy change or "icelandify" their names.

So if your name was Arthur, and you wanted to emigrate to Iceland you would you change name.

Might still be like this.