Ensure that the session (UI) locale stays the same even if receipt locale is different

Created by: mlutfy

This partially fixes an odd bug where a person might use a public contribution page in French. CiviCRM will create a new contact record and might set its preferred language to English. The receipt gets sent in English, and then the user is sent to a confirmation page in English (hence switching the UI locale).

This is only a partial fix, because if the user was on a French contribution page, we would expect the tax receipt to be also in French, no matter which language is on their contact record.

Possibly related: https://civicrm.stackexchange.com/questions/27074/contribution-page-validation-errors-display-and-confirmation-screen-switch-to-de