Faced this issue after I upgraded a Site from SharePoint 2007 to 2010.  The site was heavily customized and was using custom CSS and custom master page.  The room site language was Arabic.  As a normal case Most SP Admin guys like me here in Saudi Does not understand Arabic so went to change alternative language support.  I selected English and click OK and saw a weird error waiting for me that is mentioned above in the title.

As I designed the master page from scratch with a new CSS file I did not find any reason for this error. But even looking at ULS Logs could not find anything reasonable.  Then went on to find the article that says that you should delete the _style page if you want to enable Alternate language.  You can find it here

Even removing that folder did not fixed the issues.  I reset the CSS Link from Change Master Page setting page and set it back but nothing worked then I went on to install Language Pack SP1 and later SP1 but nothing seemed to work.  Just before I was able to Quit got an idea.  I Opened SQL Server Management Studio and went to query AllWebs Table and tried to find CustomCss column for all the webs.  I was amazed to see that the CustomizedCSS column was showing ;123#MIZAN; for the root Site Entry while all other values are null.  Although I could not find the source CSS on the site but setting the value to null and I was able to change the alternative language without any issue.