When changing languages, folder names are updated automatically in both SmarterMail and Exchange. Using SmarterMail, special folders are correctly mapped, though folders such as "Sync Issues" will still show in French. When the language is changed, it's necessary to unsubscribe from "phantom copies" of the special French folders.
Using Exchange, there is some folder duplication on the initial sync, though Inbox, Drafts and Sent Items are properly translated. When the language is changed in SmarterMail, folder names are automatically updated in the Subscribe dialog, though a restart of Thunderbird is required for the changes to take effect.
Using SmarterMail, all folders are correctly mapped in Outlook on an initial sync. However, these do not sync back to the SmarterMail server. Using SmarterMail, special folders may be duplicated in Outlook, having both a French and an English version. After the mappings have been reset, the duplicate folders can be deleted. If this happens, duplicate folders can be cleaned up in Outlook by going to Account Settings > Advanced > Folders and resetting mappings. However, we have noticed that duplicate folders can appear after Outlook has been in use for a period of time and after an undetermined number of syncs. Using SmarterMail, all folders are correctly mapped in Outlook on an initial sync as well as when the language is changed in SmarterMail. Exchange acts the same way - not initially, but after Outlook is closed and re-opened, the duplicates are synced back to the server. When changing the language in SmarterMail, Outlook automatically updates the special folder names, but the duplicates remain and the duplicates sync back to the mail server, so they will appear in webmail. Microsoft Exchange acts the similarly, except that it does sync a Drafts folder back to the server. However, these do not sync back to the SmarterMail server, so they do not appear in webmail. Therefore, after a language change is made for a user in SmarterMail, it's best to remove and re-add the account in any client that is synced to that user. IMPORTANT NOTE: Language choice, and changing that language choice, is a complex process. This is mainly how default/special folders (e.g., Sent Items) are handled as custom folders will always remain in their original language.
Using those scenarios, below is a list of email clients and how they interpret language settings during an initial sync with SmarterMail, and then what happens if the language set in webmail is changed.