I just noticed that on the User profile page concerning Notifications, all the notifications appearing in black box in the upper right corner are displayed untranslated (in English) whereas all their strings have been made available and translated for a long time ago.
(for instance "You have subscribed to this category", "You have subscribed to the forum", "Subscription status has been reset", etc.).
Moreover, when multiples notifications are displayed simultaneously, the action "close all" appear untranslated but is not localizable for the moment.
Customer support service by UserEcho