MediaWiki is really helped by the Polyglot extension. What it does is show for instance the main page in your language if there is a translation. This helps the usability of OmegaWiki and any other installation a lot when multilinguality is important.
There are a few cotchas, the first is that it prefers the ISO 630-1 code when one is available. This means en in stead of eng, fr / fra, nl / nld .. This is the same for the Babel extension. However, this extension that the change to the short version for you.
Given that the ISO 639-6 will restore the primacy of existing two character codes, it makes sense for OmegaWiki to use this algorithm.
When Kipcool had a look at the code, he found that for OmegaWiki only a few lines were really needed... If you want to see the default or English text, you can always add "&redirect=no" to the URL.