I was able to select a Welsh keyboard layout while running this app but could not get the Unicode Welsh characters to appear. (I tried the same thing with Norwegian and had no problem.) Someone suggested below that this was because it was written to Carbon APIs (in RealBasic?) rather than Cocoa, but Carbon has the Multilingual Text Engine (MLTE), which, as I understand it, handles international text just as well as its Cocoa equivalent.

Other than that, this looks like a decent and well presented app, considering it's freeware.