When a user click on the Toolbar -> Action menu, it is dynamically loaded from the server (HTTP Request).
Rendering extensions under the Export submenu are localized according to the SharePoint site language.
The problem is that the there is a static cache for the localized labels (static method GetLocalizedRenderingExtension of Microsoft.ReportingServices.SharePoint.UI.WebParts.ActionMenu class).
So the first access to this menu by any user of the SharePoint Web Application, determines the language for the Rendering Extensions labels. For example, if the site is in french, then these entries will always be in french event for reports deployed on english sites ...
Doing an IIS Reset will clear the cache and then the first access will again determine the language use for these menus.
Fix is easy: juste remove the cache, or create a cache per language.