Success in Europe*.
Game being a success in Japan or North American won't help help to have a proper EU player base on all Worlds. EU is a completely different market.
Even while XI was at its golden age with its half a million players, we were still short on EU players on some Worlds. I don't don't how is that gonna be any better with XIV.
By short I mean, there was people online off course, but it wasn't possible to do what you wanted to do because there was no [job] in [level range], or there was only ONE major linkshell running [this event] etc. Luckily there was still some JP players around, going to bed late.
I couldn't care less about language people are speaking or from where they are. All I want is people to play with when I want to play. So ideally prime time during my time zone.