Tried to write earlier, but it resulted in a message that the forum software crashed?!?
Changing the language is very easy:
1) uninstall Sins completely (may require manual cleanup for some installations)
2) use impulse to reinstall the software via internet which then will allow to select any of the available languages
Installation from the internet is much faster than from DVD (unless you've got a slow connection). I was astonished at the improved installation speed, especially since I was downloading pretty much across the world.
The only sad point is that I have not figured out a way to select the target folder/disc when installing from the net. Thus you may want to ensure that your system disc (usually C) has enough free space!