I would doubt that the course developers put bad (word-for-word) English translations in on purpose to help our understanding of Swahili grammar. (But maybe that isn't quite what you meant.)
That would be like translating "Ma mère me manque" (French) or "Mi madre me manca" (Spanish) as "My mother me misses" in English. What they actually both translate to is "I miss my mother", and it would be misleading to translate the sentence literally.
We need the translation to be the English equivalent of the Swahili sentence, i.e. what people actually say in English. Garbled English doesn't help you learn Swahili, but oversights like "the door has dirt" can act as a reminder of the Swahili structure once you already know the grammar.
So we should keep reporting that the English translation here should be "The door is dirty", so that they eventually correct the unintentional error and remove the confusion.
thanks for your input, i understand that point of view. but i think the best way to learn a language is some balance between literal word-for-word translations and meaning-dependent translations that can lose information going from one way to the other.
it would be fine if the sentence was listed as "the door is dirty" in a university-setting course because those types of courses tend to emphasize prescriptive grammar rules anyways, so the learner would understand the 'true' grammar of the sentence. but duolingo is mostly descriptive and many people do the lessons without ever looking at the 'lightbulb' notes as on mobile. it's important to know that "una" is a conjugation of "to have" and not "is", and i think a goal of language learning is to get to the point where sentence translate to meaning in your head without having to go through english translation first anyways.
also, "the door has dirtiness" is different from "my mother me misses", because the former is grammatically correct but not idiomatic, while the latter is neither. i don't think every sentence should be a literal grammatical translation, but we do need to consciously strike a balance.
Thanks for expanding on your point, giraffe_.
I know there are lots of discussions here about the question of idiomatic vs literal translations. You have probably come across several about the fact that "fruit" is uncountable in English, but the Swahili word "-tunda" is countable. I am with those who say that "matunda" should be translated as "fruit" not "fruits", because when English speakers say "two fruits", they mean two classes of fruit (e.g. oranges and lemons), not two pieces of fruit (e.g. two oranges). So that is a case where the literal translation would be misleading.
In another context (e.g. a textbook) the idomatic English would be shown with the literal translation as well, for example:
"leo kuna joto" = "it's hot today"
(literally: "today there is heat")
In Duolingo, there isn't space for that (unless it's in the lesson tips & notes) so yes, we have to take a chance that the meaning will be clear enough to the students. I would argue that by this point in the course they have probably learned to distinguish "has" from "is" in Swahili, so we don't need to give the baffling literal translation - "has dirt" - that is grammatically correct but not idiomatic English, just to ensure they notice that the word "una" means "has". (In the same way, I think "today there is heat" would baffle students. They would never guess it was a statement about the weather.)
But you are right that a Swahili course is not there to test people's English, and I have also found the English errors in the course useful when they reveal how Swahili works.