The 'torch' answer actually came about as a consequence of learner feedback. People were outraged that 'torch' was scored incorrect. It is indeed a much more beautiful word than 'flashlight'. I appreciate that the Vietnamese team recognise that a lot of us don't use American English.
Of course, and that's why Duo accepts both versions of English. By default it teaches American English though so I'm confused why the default translation is torch rather than flashlight, especially as flashlight works in both but torch is British only. I never said torch should not be accepted if that's what you think I was saying :)
Duo is not entirely consistent about wanting "the" when a classifier is used in Vietnamese but generally that is what they want. As a rule for translating it fails against English idiom at times and it often fails, as English usage, when the word in question is not the subject of the sentence. Some of the sentences in these exercises are so unlikely ever to be said that I often wonder whether the Duo translations are actually what the Vietnamese means or just bad translations.