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.
Before beginning any translation work, you should make sure you read Project:Language policy.
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).