Because of discrepancies between the actual Confluence and Language Manager usage and the frequency with which the re-index notice appeared, we decided to completely disable the notice from showing.
If you believe that rebuilding the search index is needed, you will have to trigger this operation manually from Confluence administration.
When is a re-index recommended?
We recommend to rebuild the search index (aka. "re-index") after the first activation of the Language Manager or, alternatively, after reenabling or reinstalling the Language Manager if some time has passed since it was disabled or uninstalled.
This process ensures that all language information associated with Confluence pages is up-to-date.
Usually, a re-index is not needed after Confluence has only been restarted (and the Language Manager plugin state has not changed).