The site Administrator is unable to change the Regional Settings for a site collection. Whenever he tries to do so a message appears that says something similar to the following:

“The column”Workflow History Parent Instance” in the list or library “CreateDateExpiredTasks History” has been marked for indexing. Please turn off all indexed Columns before changing the collation of this site. You may re-index those columns after the collation of the site has been changed.”

Or

“The column “Invitation” in the list or library “correlation ID number” has been marked for indexing. Please turn off all Indexed Columns before changing the collation of this site. You may re-index those columns after the collation of the site has been changed.”

Or

“The site regional settings cannot be changed because one or more columns has been marked for indexing”;

You could try to get rid of the afore mentioned column manually but that could take long time on a medium to large site.

The best way I have found so far to be able to change the Regional Settings under this condition is to make sure the “Sort Order” option is set to “General”, as in this screen-shot:

 I hope this helps.

Have a nice and productive day !