You enable languages in the configuration file
$swLanguages[] = "de";
The navigation menu will then have also a language menu. This will set the variable lang that will be saved also as cookie.
When you edit a page, the wiki will you also propose edit links for the localized pages eg Edit de. Then you can create a localized page which is like a subpage in MediaWiki with the slash syntax name/de.
On viewing the page, if the lang cookie is set, the wiki looks if there is a localized page with that name. If it is available, it will show the localized page. If not, it will show the default page.
You can transclude content of the default page into the localized page using the transclusion syntax.
Another case is when the different language pages have different names. In this case, you can add interlanguage-links at the end of the page. They will be used to set the links in the language-menu. An interlanguage link is an internal link with the lang-parameter set in front with a colon eg [ [ de:Bearbeiten ] ].
Note: Gumoz has made a spanish translation of the defaults. We will add the file to the next distribution.
Note: If you localize, do not forget to add also english as language.