I would say that your text file in encoded with some charset (iso-8859-1?) and your default system charset is different (cp1252 ?)
The best option is to set everything to UTF8.
You should read http://plantuml.com/faq
Characters encoding is a subtle thing... By default, PlantUML use the default charset of your platform, which may or may not be UTF-8 . If you want to know which charset you are using, you can type the following command:
java -jar plantuml.jar -help
-charset xxx To use a specific charset (default is windows-1252)
If you want to use a different charset, for example UTF-8, you can use the -charset flag :
java -jar plantuml.jar -charset UTF-8 ...
You can also find here information about unicode . Note that even if Java does not support B.O.M. (Byte Order Mark) in UTF-8 format file, a workaround has been put into PlantUML, so this should not be an issue.