There will be very few times in English where it would be appropriate to say "What date is this?" For e.g. in a situation where an old person with poor vision points to a written date and cannot read it themselves and has to ask. It would mean something like, "what is the date that's written here?" In English, people say "What's the date?" to mean "What's today's date?"
In English, if we're asking what the date is today (July 20, for example), there is no one fixed phrase. We can ask : What's the date? What date is this? What's the date today? What's today's date?
All ask what the date is today.
When talking about an event, for example :
"Suzi's wedding is in August." or "I want to go to the concert in the park." we can ask : What's the date ? What date is that ? What date is it ?
Here are some examples I found on-line (Reverso) in Polish. I wonder what you think of them :
What is the date today? -> Który mamy dzis dzień?
And what is the date today? -> A jaka jest dzisiaj data?
Now, then, what is the date today? -> Tak, którego dzisiaj mamy?
Który mamy dziś dzień? - Dzisiaj ? 7 listopada.
I don't think that Reverso is a safe source of getting any knowledge ;) Sometimes the English and Polish sentences have absolutely nothing in common.
I still think that I have to remove two sentences with "data" and create two that are not that context-depending. I have one right now, thinking about the second ;) This is rather early in the course so there's no much vocabulary/grammar to choose from...
I'm wary of on-line translations in general and usually consult several sources if that is my only option. The best, of course, is to be lucky enough to find a good forum with native speakers. :-)
Originally, I think before they had the web site, Reverso-Softissimo was partnered with the Collins English dictionary and dictionaries like Larousse and Langenscheidt to produce dictionaries on cd-rom for home computers, much like the existing paper dictionaries, so at least in spanish and french, the dictionaries were good. I never purchased the german one. Now on the web site they have different sections. The Collins dictionary section where it exists is still pretty good, but now they also have "collaborative" and "context" sections now, and I do think they are less reliable.
For Polish->English dictionary, I have used Pons, among others.