I think you're touching on another problem, which is that we as users rarely know why the form wants a name. Is it to be used in emails, or for sending packages, or for talking to me?

My language also has a separate vocative case, but I live in a country that has no concept of it and just vestiges of a case system. I enter my name in the nominative, which then of course looks weird if I get emails/letters from them later - they have no idea to use the vocative. If I knew the form is just for sending me emails, I'd maybe enter my name in the vocative.

Engineers, or UX designers, or whoever does this, like to pretend names are simple. They're just not (obligatory reference to the "falsehoods about names" article). There are many distinct cases for why you may want my name and they may all warrant different input.

- Name to use in letters or emails. It doesn't matter if a CSR can pronounce this if it's used in writing, it should be a name I like to see in correspondence. Maybe it's in a script unfamiliar to most CSRs, or maybe it's just a vocative form.

- Name for verbal communication. Just about anything could be appropriate depending on the circumstances. Maybe an anglicized name I think your company will be able to pronounce, maybe a name in a non-Latin script if I expect it to be understood here, maybe a name in a Latin-extended script if I know most people will still say it reasonably well intuitively. But it could also be an entirely different name from the written one if I expect the written one to be butchered.

- Name for package deliveries. If I'm ordering a package from abroad, I want my name (and address) written in my local convention - I don't care if the vendor can't read it, first the package will make its way to my country using the country and postal code identifiers, and then it should have info that makes sense to the local logistics companies, not to the seller's IT system.

- Legal name because we're entering a contract or because my ID will be checked later on for some reason.

- Machine-readable legal name for certain systems like airlines. For most of the world's population, this is not the same as the legal name but of course English-language bias means this is often overlooked.

In this specific case, it seems like your concerns are a hypothetical, no?

Sure, there are sites that mistreat names in ways you describe, but that does not make it correct.

> I do in fact want a properly functioning service to use my name in a vocative case. ... I also don't want to know to specify the correct version myself, as it's trivially derivable through established rules of the languages.

There would be nothing to discuss if this was trivial.

> Not doing that is plainly grammatically wrong and borderline impolite.

Do you know what's more than borderline impolite? Getting someone's name wrong, or even claiming that their legal name is invalid and thereby making it impossible for them to sign up.

If getting a name right and using a grammatical form are mutually exclusive, there is no argument to be had about which to prioritize.

Absolutely not. There's no way to understand what a source user's reading capability is. There's no way to understand how a person will pronounce their name by simply reading it, this only works for common names.

And here we go again, engineers expecting the world should behave fitting their framework du jour. Unfortunately, the real world doesn't care about our engineering bubble and goes on with life - where you can be called !xóõ Kxau or ꦱꦭꦪꦤ or X Æ A-12.