OK, since I last posted about my problems with screen and irssi being unable to handle unicode chars, I got a lot of feedback (here or on IRC), and actually it was Alexander who pointed me into the right direction. LANG=C doesn’t seem to support UTF-8 characters. So after adding

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export LC_ALL="en_US.UTF8"
export LANG="en_US.UTF8"

to my environment everything is just fine … 😳