Hi! I’ve been to Croatia twice in the last few years and the staff at the post office always spoke at least a bit of English so I assume there will be no language barriers (at least in areas with more tourists).
There are stamps with their value printed on them, but there are also NVI (no value indicated) stamps. B or C rate stamps are sufficient for an international postcard (they both cost the same — 8,60 Kuna when I was there and I think they just converted the old prices into Euro currency).
This is how the postcard rate stamps looked like last year:
