When using non-US character sets (including accented characters, characters with umlauts, etc) make sure that you save the file using the UTF8 character encoding.
A Better Finder Rename supports the entire unicode character sets and should thus be able to cope with all possible symbols and writing systems.
Using non-US characters within plain text files is, however, a complicated topic in its own right. If you experience problems, take a minute to read the information on unicode (http://en.wikipedia.org/wiki/Unicode) and UTF8 below:
http://en.wikipedia.org/wiki/UTF-8
http://en.wikipedia.org/wiki/Unicode
How exactly you go about producing UTF8 encoded plain text files from your particular text editor, word processor or spreadsheet is beyond the scope of this documentation, but should be covered in the program’s own documentation.