Originally Posted by Marius_Hancu
an important factor:
large Spanish-speaking world vs. small country (Serbia)
It's the player rather than the place or language.
Population of serbia is approximately equal to that of switzerland (7m) and both are small countries. Number of swiss-german speakers is perhaps even smaller. Both Djok and Fed speak English well, better than Nadal certainly. But both Fed and Nadal are very popular.
Jerzy Janowicz - polish, probably spoken by fewer than spanish, but he's quite popular, atleast on these boards. Going back further, I am guessing Edberg was also a swedish speaker, but certainly a popular/respected athlete.
I haven't looked up each of these to confirm, but broader point is that it's the player that determines the popularity.