This is a weird issue I have with digikam since a long time:
I typically use the “UK extended, Windows” keyboard layout (not sure if it is called exactly like this in English, the German label is “Englisch (Britisch erweitert, Windows)”). When typing in German with this layout, I can write umlauts (äöü) by pressing AltGr+2, a/o/u. This works literally everywhere except, for some reason, in digikam.
Does anyone know why it doesn’t work and how I can make it work? Currently, I always switch to German layout if I need umlauts in image descriptions, which is a bit annoying as I then all the time mistype Y and Z 
).