Hi, 
i have the same problem with some german letters.
But i also have a program named "PS Pad Editor".
I don't test it til now but i think this is good.
You can search and replace -> so i search in all(!!!) Files in the Device
("Ordner" in german...), sorry for my bad english, and (write) "search for
ä" <- it looks very crazy... and "rename to ä" - i hope that is OK.