Ration one's expressions of appreciation when abroad? That's barbarous! (9)
I believe the answer is:
merciless
'that's barbarous' is the definition.
(thesaurus)
I can't explain the rest of the clue.
(Other definitions for merciless that I've seen before include "Ruthless" , "Totally unforgiving" , "Uncharitable" , "Showing no compassion" , "Cruel" .)