Ok, well as I said it works here with DejaVu fonts (I don't have courier) and it works with both 1.36 and 1.39 (current git). And we have exactly the same version of GTK.
Thanks for trying the encoding, but as you found, it is unlikely to have any effect because the Geany buffer is always UTF-8 no matter the file encoding. It might impact the ability to save the characters though.
So the things that are different are that its a very old Geany at 1.34 although I am not sure that was so terribly different to 1.36 which works here. Is it possible for you to try a newer version? I thought someone was building Geany on the Suse Open Build Service but maybe they stopped.
Otherwise its something about Suse vs Mint, IIUC Suse is Qt/KDE based and Mint is GTK/Gnome based so maybe there is something happening in the Suse backends behind GTK. Not sure how you could track it down.