About [[MediaWiki:Revcs-alert-exist-supplement/en]]
Typo, "exist" → "exists".
As I told you on #mediawiki, I'm assigning these requests to yourself. Add Siebrand and mention that it's a typo in first line of commit message and I'm sure you'll get quick merge.
Actually, what is the reason why one can't edit English-language messages on here? This would be useful in cases like these.
Because software developers usually want to be in control of their software: customising and changing the interface is quite different a task than translating it, it usually should not be equally easy. Besides, TWN and Translate don't support only MediaWiki and even MediaWiki development used not to be so tightly attached to TWN community.