The kdevelop editor's default font ("Arial") does not support the euro symbol.
i cannot reproduce it, it works for me. Please check if you LANG in /etc/sysconfig/i18n is correct. here is my i18n and works. LANG="de_DE@euro" SUPPORTED="de_DE@euro:de_DE:de" SYSFONT="lat0-sun16" SYSFONTACM="iso15
Quite similar: LANG="de_AT@euro" SUPPORTED="en_US.iso885915:en_US:en:de_AT@euro:de_AT:de:de_DE@euro:de_DE:de" SYSFONT="lat0-sun16" SYSFONTACM="iso15" Very similar, I'd say. I even did not use LC_MESSAGES to get rid of that german translations ... that's real self-sacrifice ... ;) I'll try to use your config and see if the problem persists. However, I think it's the fonts installed. Could you give a list of the font packages installed on your test system? I'm using an everything install.
I suppose it was the fonts. I upgraded to beta4 with all the packages available through RHN. Things work fine now.