"Mówię do widzenia."
Translation:I am saying goodbye.
The case "angielsku" (spelled with lower case, unlike in English, because it is an adjective) exists only with the preposition "po". It is a relict from Old Slavonic. It is a part of a prepositional expression with "po":
po - po co, po cóż, po części, po czym, po drugie, po dwadzieścia, po hiszpańsku, po kolei, po omacku, po południu, po prawdzie, po prostu, po staremu, po swojemu, po szlachecku, po trochu, po troszkę, po trzecie, po wszystkim;
It is the plural form of "widzenie" and can be the noun form of 'to see' (i.e. "Widzenie w ciemności" - 'Seeing in the dark' / 'The ability to see in the dark') or can mean something like 'a short visit', used for instance in the context of jail - the hour the inmate can spend talking to their friends / family.
The meaning comes from the whole phrase, not its separate words.
Do widzenia - Goodbye/(Until we see each other later or next time)
The sentence reports "what I say", so it should look like a citation:
Mówię: ,,Do widzenia" - I say: "Goodbye"/ I am saying: "Goodbye"
The phrase: "Mówię do (kogo? czego?) widzenia" does not make any
sense, since "Mówię do..." means: "I talk to somebody or something":
Mówię do ciebie/ Mówię do was - I talk to you/ I am talking to you
Mówię do ściany! - I talk/ I am talking to the wall (Nobody listens!)
Aside from not being able to use Polish quotation marks in a locked tree, because they were not defined as separate lexemes, or not really wanting to constantly switch between English/Polish keyboards, the grading program does not assess the apostrophes and quotes properly (it's buggy).
For instance, your ,,Do widzenia" has a different font for opening and closing quotation marks, so it will be impossible to train Duolingo program to recognize them as a pair.
English opening and closing quotation marks should in fact be different, with inverted commas in the front. I don't even know how to type them on my computer and in the sans serif fonts the dot in a comma is lost any way, though the tick direction should be different... this is simply not a technically feasible feature to introduce.
In addition, as I pointed out in the last comment ("Such a sad sentence . . .") on this page, "Mówię do widzenia" is often said literally at the end of a party to a hostess/host to let them know we are leaving. If there were a colon, we would have to read it with a hesitation before the quote, but we are really pronouncing it in one breath.
Chodzi o to, że vzGH4 początkowo myślała, że zdanie:
Mówię: ,,Do widzenia" jest w trybie rozkazującym.
Dowiedziała się, że nie jest, ale nadal chciała wiedzieć,
jak można kogoś poprosić (nakazać, rozkazać), żeby powiedział: ,,Do widzenia" (tryb rozkazujący).
Mówię: ,,Do widzenia" - I say/ I am saying: "Goodbye"
Powiedz/ Powiedzcie: ,,Do widzenia" - Say: "Goodbye"
It's actually very easy once you get used to it. I would argue considerably easier than that of my native English. It's just different from English's assumptions/expectations, making it a wee tad tricky, but objectively speaking it's really simple, with loads of predictable patterns.
Not in Polish, it is often said literally at the end of a party to a hostess/host to let them know you are leaving. In Polish it has an additional meaning "see you next time," so it is more hopeful than "a farewell".
BTW: If you did not say goodbye to the host and left we would say Ulotniłem się, literally I evaporated or Wyszedłem po angielsku, literally I left the English way, which English people call, I believe, "the French way."