Because translations are updated continuously, by multiple people, so there is no 'previous version', at least if I were to restore the version used by the last calibre release for instance, it would possibly wipe out other, possibly desirable, changes to the file. And because the translation files include line numbers, trying to use a diff tool on them is way too painful.
As for mistakes in translations, you are welcome to correct them yourself, see the instructions here:
http://calibre-ebook.com/get-involved
I try to stay away from translations as much as possible myself, I have quite enough on my plate as it is.