this is the first time i've ever heard of this word "opinionatedness". Heck, even Google marked red, lol.
It made me reach for the dictionary too. And lo and behold it IS there.
Ahhh Duolingo - it's a two-way learning street
It would be much better to translate this literally as "hard headed" or even "head strong". Those are all much more common English expressions, and they reflect the imagery implicit in the Vietnamese expression
that is true but I feel I've heard cứng đầu used more often in the sense that a person is stubborn, rather than that a person is practical and unsentimental