Here, people translating the MediaWiki website can coordinate their work. Note that this page and its sub-pages relate only to the translation of MediaWiki.org. To help with translations of the MediaWiki software itself, please visit translatewiki.net.
One thing to be aware of is that when translating a page, any 'category' links in the page should be modified to also include the appropriate language code. For example, if you translate a page from English to French (fr) and the page contains the tag [[Category:Policy|Language policy]], you need to replace it with [[Category:Policy/fr|Language policy]]. If this results in a red link, you will probably want to follow the link and translate the category page as well (which should be quick - most are blank except for containing a parent category).