Found a Serbian flag sr.svg file (in an old version of w34) to replace the current sr.png. Unzip, add it to /img/flags/ directory and change line 149 of w34_menu.php from
<a href="index.php?lang=sr"><img src="img/flags/sr.png" title="Serbian" width="25px" height="15px"></a>
to
<a href="index.php?lang=sr"><img src="img/flags/sr.svg" title="Serbian" width="25px" height="25px"></a>
I presume this (or something similar) would work in beta and the new release.