I don't know of a way to reflash the official Sony firmware on the 300, since it doesn't have an SD card (and everything I've seen refers to using the SD card).
However, if you're willing to have a modified Sony 300, using an alternative firmware, you might want to check out the
alternate Sony software thread.
Otherwise, I hope someone else here can help. I think the member
porkupan is the fellow who's "russified" various Sony versions and he might have a good answer for you.