Translation:I want a cake with a lot of cream on.
That’s not a real rule of English grammar, and never has been! It was suggested by 18th-century grammarians who tried to analyse English after the style of Latin, and from there, it got into schoolbooks, from which it stubbornly refuses to be dislodged. But no serious linguist has accepted it for at least a hundred years, and few serious style guides have even proposed it as a style guideline. Here’s an Oxford Dictionaries takedown.
I tried, “I want a cake with a lot of frosting.” I think that is the right meaning and should have been accepted. Where I live, I have never ever heard whipped cream referred to as cream. Cream also means heavy cream or half and half. It would be weird to pour either of those on cake.
There's a lot of local variation within English, but the thing they call 'frosting' in Standard English (see e.g. https://www.collinsdictionary.com/dictionary/english/frosting) is glasyr in Swedish. We should rewrite or remove this sentence but the grädde in it definitely refers to 'whipped cream' (a.k.a. vispgrädde), nothing else.
The Swedish version of Duolingo has so few problems with the English translations that it really shouldn't be receiving complaints, and they usually get around to fixing ones that get through. I could perhaps see some more reason for complaints about the Russian or Spanish which you are also working on, but even there the difficulties seem mostly to be with the automation of the process of creating translation pairs (needed to make the whole thing free) rather than a lack of bilingual fluency on the part of the creators.
I think in English (Hiberno if not British) you would either need the verb 'put' or pronoun 'it' for this sentence to be idiomatic. Otherwise you'd just drop the 'on'.
In my opinion, the addition of 'put' sounds best.
- I want a cake with a lot of cream put on.
Ending a sentence with a preposition is fine, it is just informal, as is this sentence in every context.
Is en tårta equivalent to a torte (a cake made from crumb rather than flour, usually very rich with cream and berries)? I was just curious if it is a specific type of cake. In English, we have numerous desserts that seem to get glossed over as cakes or pies. Sometimes the nuances are illuminating. For example, my one English friend always used to joke that there is no such thing as a cherry tart... I didn't get it until years later.
A proper English speaker would never translate this sentence with "on" at the end. Never. It makes no sense. Moreover, in English, cream is liquid; what is needed here for proper translation would be "whipped cream." So "...whipped cream on top" or "whipped cream on top of it," or "whipped cream on it" would be used. Again, never "cream on."