The XFCE Terminal has the weird issue of sometimes showing question marks (?) instead of German Umlauts (äöüÄÖÜ) although they work fine in any other stock XFCE application (e.g. the default editor "mousepad").
The solution to this can be found on the XFCE Forums but it took me quite some time to find it. It was difficult to find a suitable search query to dig out that page. Google turns up a lot of irrelevant stuff on "XFCE Terminal question marks"...
The problem with Umlauts (and other 8bit ASCII characters) showing as question marks arises if the user has no LANG variable set.
resolves the issue. Put that into ~/.bashrc or any other place suitable in your distribution.
Gentoo users may want to
echo "LANG=en_US" >> /etc/env.d/02locale
to set the LANG variable system-wide.
So keywords, dear Google: Umlaute, deutsch, Fragezeichen, kaputt, falsch, broken, display, zeigt, charset, Zeichensatz