We have a solution running in the danish language layer. After upgrading the solution to Sitecore 8.1 update 1 the site defaulted to english, the siteresolving seemed off and the link manager rendered links to a api website instead of the website that serves the content as HTML.
After researching a bit I discovered that Sitecore have introduced a new Sitecore.DefaultLanguage.config file.
Disabling this file solved the problem.