I'm opening "Font Preferences", then change e.g. "Application Font". I get the
"Pick a Font" dialog, and in the Preview area there, I see the lowercase and
uppercase letters.

This alone might not be enough for me to choose a font, however, I want to see
if it supports Umlauts (something to be added by the translators), and I want to
 see some numbers, and the Euro sign, the question mark etc.

In the german translation for Inkscape, I put the following string in a similar
dialog: 

AaBbCcIiPpQqÄäÖöÜüß012369€¢?&.;/|()„“»«

The LANG=C default should probably not have the German Umlauts, but some or all
of the others. There is some background in
http://sourceforge.net/tracker/index.php?func=detail&aid=1240559&group_id=93438&atid=604308

Cheers,
  Colin

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