Thanks for the quick and detailed explanation, it didn't help with none of both issues.
Code:
mkvmerge.exe -h -ui-language en -> Gespr"chigkeit erh"hen
mkvmerge.exe -h -ui-language en --output-charset en -> Gesprächigkeit erhöhen
edit: I'm on Win 7 32bit english with german locale, if it helps you I can boot into XP german and check if it behaves different.