[gtkmm] Using German umlauts in GTKMM buttons, labels etc.

[Markus Gerwinski <markus gerwinski de>]

Hi folks,

when providing a button, label or similar with a text containing a German
umlaut, e.g. "Löschen", I get a "WARNING: Invalid input string" error message,
and the text isn't displayed on the screen.

Can someone tell me how to fix this?

Thanks in advance,

  Markus