The ability to set the Main Language for content, in the Translation tab of the admin interface, when no Prioritized Languages exist, does not work.
It's stated in the documentation that "When an object doesn't exist in any of the site/prioritized languages and it is always available, the system will use the object's initial/main language to display its contents." ( http://doc.ez.no/eZ-Publish/Technical-manual/4.x/Features/Multi-language#always_available )
However, when using multiple languages (with each language via a different siteaccess) with ShowUntranslatedObjects=enabled set (in site.ini > RegionalSettings) and an empty SiteLanguageList, and AlwaysAvailable checked, this doesn't work at all.
What happens is the available translations are sorted alphabetically by LanguageName, and the top one is used instead.
See http://share.ez.no/forums/developer/always-available-not-always-working/ for further details.