Pierre-Luc, thank you for bringing this issue up. It is possible to create and sync folders with foreign characters (accents) via EAS in Outlook. However, when looking into the issue, I noticed the Outlook appends [1] to the end of those folders. Renaming the folder in Outlook and syncing the change does, however, break the SmarterMail web interface after the sync. This is caused by SM failing to encode the folder name in the file structure.
It is currently being fixed.