charset question (Tajikistan iw-links)

Hello!

I encountered rather unusual situation when I tried to add interwiki sl: 
link to en:Tajikistan <http://en.wikipedia.org/wiki/Tajikistan>.

I noticed that Estonian and Slovenian use the same word (Tadžikistan) so 
I tried to copy the exact [[et:Tadžikistan]] to [[sl:Tadžikistan]].  But 
to my surprise this does not work in sl:, because it wants to open 
http://sl.wikipedia.org/wiki/Tad%C2%9Eikistan while the correct link is 
http://sl.wikipedia.org/wiki/Tad%C5%BEikistan.  The Estonian version has 
no such problems: http://et.wikipedia.org/wiki/Tad%C5%BEikistan

Could someone, please, explain how this is possible?
-- 
	Pozdrav,
	  Roman