I found the reason:
The file was saved by eclipse default encoding (Cp1252). I changed it to UTF-8. Now, every file is saved in UTF-8 (I hope, there are no other negative side-effects of my source-code when using UTF-8 instead of Cp1252), and with <meta charset="UTF-8" /> it works.
So the reason was not a missing UTF-8-setting in Jboss AS Container but the file was not saved in the correct format.