Thus So if you have at you computer files from many different sources, you never know, which letter is used in certain filename. In order to find all files with the two words "Köln" and "Zürich" you have to make many separate searches one by one: for "Köln" and "Zürich", than "Koln" and "Zurich", than "Köln" and "Zurich" and so on

(Well, you can use the pipe or regular expressions, but for a query with several accented words the syntax of the query will be very, very sophisticated.)
The solution is obvious: internal replacement by the mask as O = Ö = Õ = Ø etc.
Some other good file managers with multi language support has the proper feature (say, XYplorer http://www.xyplorer.com/index.php).
TC great plugin QuickSearch eXtended (search and filter inside the pane) by Samuel has such option too - see http://ghisler.ch/board/viewtopic.php?t=24288.
But this option is extremely needed for regular (global) search by Alt+F7.