Translate to Spanish
thanks a lot for you effort. You approach sounds reasonable. Indeed, pages already marked for translation are priority. In case there is something to ask do so ...
The individual semantic extensions for translating the system messages can be chosen from the dropdown menu on this page, so yes there is a centralized website to do so. This one uses the same translating system as this wiki and most of the other websites running MediaWiki.
Do not worry about signatures. :)