Because I'm guessing the term "hard-R" only makes sense in some sociolinguistic US accents. As an outsider I can't really have an opinion. As an NZer I can say that unfortunately we sometimes get judged according to US language rules in some contexts - so the rules affect us so it sometimes helps me to know US practice.

My comment explains what hard-R means from the point of view of someone outside the states, and gives enough context for a non-native English speaker to understand the term. The subtleties of English are hard even for those with English as a mother tongue.

From the Wikipedia article:

  Among certain speakers, like some in the northeastern coastal and southern United States,[6][2] rhoticity is a sociolinguistic variable: postvocalic /r/ is deleted depending on an array of social factors,[7] such as being more correlated in the 21st century with lower socioeconomic status, greater age, particular ethnic identities, and informal speaking contexts.