Also interesting having a $2 for commons, ie. http://commons.wikimedia.org/wiki/$1?uselang=$2
What we want here is some configuration so you would have an optional $2 parameter selecting the project.
How do you set the dafault value for $2? Using a default value of $wgContentLanguage would work for us, but is not optimal. Some odd domains are not equal to the language code (eg: $wgContentLanguage of simple is en). Plus, some external URLs may not need a language code, or only some of them. Wikitravel uses format http://wikitravel.org/$2/$1 but if it had a page named Sh:sh you would end up at http://wikitravel.org/sh/sh (requested URL was not found on this server).