It's not an error as such, but to me "Which date is it today?" only really sounds natural if the date is being identified from a predetermined list. If you're simply asking for today's date, I would find "What date is it today?" much more natural (or "What is the date today?", or "What is today's date?").
Does the same distinction exist in Swedish, or would this be the standard way of asking for the date? I notice that Swedish does often seem to use vilken/vilket where English would use what.
In principle, vad för is more like what and vilken is more like which. I'd say the most idiomatic way of asking in Swedish is Vad är det för datum i dag? (and I got more hits for that in the corpora I just searched)
For one thing, it may be as you say that we tend to use vilken a little more often than you use which in English. Here, it may also be that it seems easier to teach the structure with vilken so that we overuse it a bit because of that.
So while I definitely think 'which date is it' sounds less normal in English than Vilket datum är det does in Swedish, I still kind of think which might be the best main translation.
This raises a really interesting point. There's tension I've noticed in all the language lessons I've done on Duolingo.
On one hand the literal(-ish) translations are useful in learning the structure of the language.
Vilket datum är det i dag? > Which date is it today?
On the other hand, speakers of different languages normally phrase things differently, and using the normal phrasing has a meaning, too. (Less likely to be misunderstood, among other things.)
Vilket datum är det i dag? > What's the date today?
Jag vill ha fredag i dag. > I want it to be Friday. (Notice natural English omits 'today'.)
- In Swedish that would be: "Vilken dag är det i dag.".
- Words 'day' and 'date' generally mark different things, thus not being fully interchangeable - see below.
《 Day = Dag 》Name of the day [in general], e.g:
- "It's Monday today."
- "Det är måndag i dag."; etc.
《 Date = Datum 》Number of the day [+month/+year], e.g:
- "It's the second of January."
- "Det är den andra av januari."; etc.
vad on its own cannot be used like you use 'which' in English – in order for it to work that way, you have to add för too: Vad är det för datum? = 'What date is it?'
But you can use vad like you use 'what' in the cases where 'which' wouldn't work in English, e.g. Vad är det? = 'What is that?'