I'd like to see german language support in festival, but don't know whether there might be copyright restrictions.
According to my information the German language is currently not supported by Festival (currently multi-lingual support for English and Spanish). Extending festival with new languages is something that needs to be done upstream - please contact the Festival people at CSTR for this.
Nope. It just has to be compiled as an additional module. Sorry, I forgot to mention that source: http://docs.kde.org/en/HEAD/kdeaccessibility/kttsd/configuration.html
mbrola has german in it now, so emerge the new festival ebuild with USE=mbrola. Also, check out how the spanish festvox voices were added, and submit a patch if you meant festvox voices rather than mbrola.