Do you have an example? I just take a quick look to Czech on osm.org, but haven't found such parts in names.
I would suggest that you have to use the original string, because they are just compared.

Henning

Am 06.06.2013 19:40, schrieb Felix Hartmann:

In Slovakia as well as Czech Republich - just like in Germany - there 
are some prefixes for cities - which shouldn't be in the name - however 
they have special symbols, can someone have a look, why the following 
doesn't work?:
(the substitution of neither Okres, nor Kraj works) - both words have 
some special accents....

( mkgmap:country=SVK | mkgmap:country=CZE ) & mkgmap:city!=* & 
mkgmap:admin_level8=* { set 
mkgmap:city='${mkgmap:admin_level8|subst:Okres |subst:Kraj}' }
( mkgmap:country=SVK | mkgmap:country=CZE ) & mkgmap:city!=* & 
mkgmap:admin_level7=* { set 
mkgmap:city='${mkgmap:admin_level7|subst:Okres |subst:Kraj}' }
( mkgmap:country=SVK | mkgmap:country=CZE ) & mkgmap:city!=* & 
mkgmap:admin_level6=* { set 
mkgmap:city='${mkgmap:admin_level6|subst:Okres |subst:Kraj}' }
( mkgmap:country=SVK | mkgmap:country=CZE ) & mkgmap:city!=* & 
mkgmap:admin_level9=* { set 
mkgmap:city='${mkgmap:admin_level9|subst:Okres |subst:Kraj}' }
( mkgmap:country=SVK | mkgmap:country=CZE ) & mkgmap:city!=* & 
mkgmap:admin_level10=* { set 
mkgmap:city='${mkgmap:admin_level10|subst:Okres |subst:Kraj}' }