Most of the language files can be fiddled with at betawiki, as well as locally on the wikis.(Betawikis is nice, since the changes there will eventually be integrated into the MediaWiki software and used in every wiki in your language that uses the MediaWiki software. Changing the actual name of the namespace, however, can only be done by developers.
The recommended way, as I understand it, is to first have a discussion on your wiki to reach a decision and then file a request to have the namespace renamed at Mediazilla http://bugzilla.wikimedia.org/ You can also join one of the IRC-channels with tech folks in it and ask to have it done, but you should be able to show a concensus on your wiki first.
/habj