"Which shop is the wheat flour available?"
Translation:Unga wa ngano unapatikana kwa duka lipi?
The English sentence is incorrect grammar. Even though it's rather easy to guess at the meaning. written like this it makes no sense. It should be, "Wheat flour is available in which shop?" Or, if you insist on inversion, "In which shop is wheat flour available?" In is not optional. Normally this would be phrased, "Which shop carries/has wheat flour?"
I agree. If this was rephrased to be a statement it would be "wheat flour is available in/at the shop". It would never be "wheat flour is available the shop" because that is completely incorrect and would not be understood. This also applies to the question. It needs a preposition before "shop". Even though its meaning can still be understood, it is wrong and should be "in/at which shop is wheat flour available".
But if you ever talk to Africans that speak English that they learned in there country (and I have, a lot), they always leave out the prepositions, which is majorly annoying to ...Americans, anyway. I was going to say native English speakers, but since it IS one of their native languages, it doesn't really apply. Anyway, I assume their way of speaking English is a carryover from their other native languages, in this case Swahili, but I have spoken with so many Nigerians, and with different tribal languages, and they are the same in regards to omitting the prepositions. So I think this is something we just need to accept in learning this language...but it really would be helpful if the English translation were correct, I agree.