I'm using rainlendar2 with Ubuntu 7.10 and I fixed this problem as follows
(I guess you installed rainlendar2 with the *.deb-file):
- exit rainlendar2, if it is already running
- down load the language-file (*.r2lang) for the languge you like, from the internet
- copy the file - with root rights - into the folder: '/usr/share/rainlendar2/locale'
- start rainlendar2 again and you should find the new language.
I used 'Krusedar' for this job, but think of the root rights.