The default language is document by document. The first content created for a wiki page name is the default document. It's language is by default the language that the current user was having setup on xwiki when the document was created.
The language can be changed using "More Actions"/"Edit Content" and change the language in the form below the main content edit box.
When changing this language you should make sure that there are no translations being setup for this language (delete them before).
Documents presented to users are selected based on the language setup in their browser or using a cookie (the language cookie changes when you click on the language links at the top right).
In the preferences you can decide to force a unique language for your wiki (make sure there are no translations of your documents before doing this) and choose the language of your wiki.