Today, when I tried to change the locale settings for my public facing website, I received the following error.

The column “Site Element Id” in the list or library “Site Elements” has been marked for indexing

To work around this, just do as you’re told.

Open settings for the Site Elements list. Open the Site Elements Id column, set “Enforce unique values” to No, and click OK.

Back on the settings page, click Indexed columns, then click Site Elements Id, and then click Delete.

Go back to Regional Settings under Site Settings, and try to change the Locale settings again. This time it should work.

Remember to reconfigure the indexed column, and set “Enforce unique values” to Yes on the Site Element Id column.