When you compare the localisation of these closely related languages, I find that the Serbian language is not keeping up with the others. In a way it is not that easy because there is both the Latin and the Cyrillic script that is to be maintained. The number of articles would have it that the amount of attention the Serbian Wikipedia is getting is much bigger.
Localisation is one of the most important usability factors. It is one of the few things that our communities can do to improve the user experience; localisation and writing the best articles. The rest is in the hands of the developers.