The PlantUML language use letters to define actor, usecase and so on. But letters are not only A-Z latin characters, it could be any kind of letter from any language.
The default charset used when reading the text files containing the UML text description is system dependent. Normally, it should just be fine, but in some case, you may want to the use another charset. For example, with the command line:
Or, with the ant task:
Depending of your Java installation, the following charset should be available:
On PlantUML diagram, you can integrate: