Translation:We want to know where the mother of the seven boys is.
I thought you use the affirmative word order in reported questions, isn't that true? https://learnenglish.britishcouncil.org/en/quick-grammar/reported-questions
There are already many reports at this question suggesting the same as you so probably something must be done.
Well, I am from Canada, and to be honest if you asked me to explain the grammar I learned in school now I doubt I would be able to. I just wrote the answer as I would say it naturally without thinking. Possibly not even technically correct grammatically.
I'm not saying that the response listed is not correct, just that this one could be added.
It is not a reported question -- it is raising a question. Inversion (subject-object here) doesn't only happen in questions. That just happens to be all they teach us anymore. It could have to do with locative inversion ( https://en.wikipedia.org/wiki/Subject%E2%80%93verb_inversion_in_English#Locative_inversion ), say. It happens in conditional statements. On top of this "where" is a funny word that acts more like an adverb than other W/H words, as if it means "to what place" with the "to" in it, like it is dative or something. Basically, when it comes down to it, all you can do is trust your instincts -- it is certainly beyond me. English can be strange . . .
The way Duo has it is more common though. That is what makes it standard more than correct, though that be a myopic view of the rich history of any language . . .
I think that any.of the suggested alternatives would be perfectly acceptable in colloquial English, though they're but quite as correct. As the purpose of this app is to teach Czech, not the nuances of English grammar, i think they should be accepted or at most treated as typos. Native English speaker and English teacher here
For what it's worth, I am not a fan of "... where is the mother..." in this sentence. It strikes me as a too-literal and somewhat clunky translation of the Czech sentence. I could perhaps see it in a particular situation. Maybe you've already asked, "Where is their mother?" and you did not get an answer. Now you're annoyed, and you say very emphatically, "I want to know, where IS the mother of those boys?!?!" There are very few reports for this word order in the system, and I would not recommend adding it as an alternative at this time.
----- UPDATE 26 Jun 19 -----
There is (currently) no Reply button, so I will address the comment from LossSS here. We have native English speakers "voting" for both "where IS the mother" and "where the mother IS." In a question, "Where IS the mother" is perfectly fine. In a declarative sentence like this, "where IS the mother" is not perfectly fine. If someone used it, s/he would almost certainly be understood -- but that word order is at least unnatural, and dsarkarati has given a very nice grammatical explanation for why "where is the mother" is not appropriate here.
As a native english speaker and ex grammar school i would have thought using "is" after "where" would require a question mark asking "where is the mother" while at the end of phrase "is" indicates a statement of fact, what i wish to know and not asking a question. The sublety of this is unusual in normal standard english.
The translation is completely correct. "where the mother of the seven boys is" is a noun clause beginning with a question word. The question word must be followed by the subject of the clause and then the verb of the clause.
Here are some other examples: Where is she? I don't know where she is. What time is it? I want to know what time it is. Why isn't he here? I don't know why he isn't here. Where did he go? Nobody knows where he went.