Locale/Encoding problem in menus

[Jeffrey Ratcliffe <jeffrey ratcliffe gmail com>]

A couple of gscan2pdf users have a locale/encoding problem with menus
in gscan2pdf in French with KDE 4 & 5:

https://sourceforge.net/p/gscan2pdf/bugs/230/

To summarise, non-ASCII characters in menus are replaced by a black
diamond/rhombus with a question mark inside:

https://sourceforge.net/p/gscan2pdf/bugs/_discuss/thread/aa2fd1b1/1190/attachment/Screenshot_20161031_155115.png

Needless to say, I cannot reproduce the problem on my machines with
MATE or Cinnamon. However, I do not have a machine with KDE 4 or 5 to
test on.

But, as gscan2pdf has been translated into 34 languages, most of which
use non-ASCII characters, I feel this cannot be a problem with
gscan2pdf, and is likely a bug or misconfiguration somewhere else, or
I would have more than two users with reports like this.

The problem doesn't seem to affect all GTK applications, as they say GIMP is OK.

Does anyone have any insight on what might be causing this?

Regards

Jeff