When changing a users language, either from the administrative interface or from the user's settings page, SmarterMail checks for potential folder name conflicts. For example, an account that is configured to English will have the default folders Inbox, Drafts, Deleted Items, Junk Email, and Sent Items. If the user creates a folder named "Courrier Indésirable", which is the language string for Junk Email in French, and then tries to change their language to French, SmarterMail will prevent the change because it would cause a folder name conflict.
Propagating the language setting to users would make it difficult to properly implement the same checks and notify the administrator which users were not properly updated due to these conflicts. Because of these complications, we decided to not allow propagating the Language setting to existing users.