Even most native English speakers say "least" these days but it is still contentious so "fewest" more correct with countable nouns like "people" and "least" should be used only with uncountable nouns like "milk".
On all the Duolingo courses I do, many people in the forums are very insistent on the most correct English. That seems to be less in the Vietnamese course.
My view is that "least" should be accepted because even native speakers say it, but "fewest" should be the default answer because it's still regarded as wrong in the English-speaking world just like "ain't" and "gonna" etc.
I teach English and didn't realise this. I never use 'fewest' in every day speach. I guess, because the comparative 'less' is used with uncountable nouns then it figures that the superlative 'least' should be, too. However, "There is less..." is commonly used for uncountable nouns but "There are less.." is used for countable nouns. Therefore, it follows that the superlative 'least' should be fine to use with either......?