my MediaWiki doesn't understand Umlauts

when i search or try to add an article with the inputbox
that includes 
Umlauts such as ä, ö, ü i get a blank page as result.
when i use code %C3%A instead of ä e.g. in "Sven
Väth" as "Sven V%C3%A4th" i 
get:

Bad title
The requested page title was invalid, empty, or an
incorrectly linked 
inter-language or inter-wiki title. It may contain one more
characters which 
cannot be used in titles.

How can i fix that?
I hope there is an easy fix for it so i can use the umlauts
right in the box 
when searching or creating. 

cheers

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