I dont like that you are using pixel definitions and that you are using visibility:hidden. Also I think that it's not the most intelligent to address each language seperately.
What about using the pseudo-class :not() ? You can hide the other languages by using display:hidden, but maybe you should not just hide them, because it also gives you some information in how many languages something is referred to, but you could make them semi-transparent:
#p-lang li:not(.interwiki-en):not(.interwiki-fr):not(.interwiki-de):not(.interwiki-eo) a {-moz-opacity:0.4 !important;}
medjakov said about Wikipedia - Delete languages on Nov 06 2008
John Arnbakle said about Wikipedia - Delete languages on Sep 21 2008
matrixik said about Wikipedia - Delete languages on Nov 18 2007