New " Select language" box has usablility problems.
The new " Select language" box runs out of the current window (viweport) by quite some length and apears not to be scrolleable. Neither does it move with with the window content (opposite, e.g. to the ajax editor windows) nor does it allow to "grabbed" and dragged around by mouse movements like ordinary windows,nor can you shrink it like you could with ordinary windows.
Thus, you're out of luck. When your screen is not of the rather huge kind, you would not see the lower end, and be unable to select one of the languages down hidden there.
It is in development and doesn't yet match the design. You can leave feedback at http://www.mediawiki.org/wiki/Talk:Universal_Language_Selector