I get the following error: Please turn off all indexed columns before changing the collation of this site. Once or more list or library currently contains a column that has been marked for indexing. You may re-index those columns after the collation of the site has been changed.
Each site represents a site collection and regional settings are selected for each course. The error tells you that you need to disable all indexes to switch the locale. The Mailbox field 'From' is indexed, so you should either disable a Mailbox feature, or remove index from there via UI.