For those connecting onward to a oneworld flight, the JL Sakura lounge is accessible as long as you are in the terminal area for it. (
It does have showers. (Modified with
supergrandslam's information.)
Admirals Club members (either thirty day membership with receipt tendered as proof, or annual members with card) may access the Sakura Lounge - as long as they are departing on AA metal, regardless of cabin class, with up to two guests. A one day pass does not provide admission.
Admirals Club member plus two guests or immediate family. Must present boarding pass and Admirals Club membership card for same day travel on AA marketed and operated flight*. Not valid for arriving guests.
The Sakura Lounge is
not available as an Arrivals Lounge if one is not continuing on as a oneworld operated flight passenger.
There will not be a new AA / Admirals Club at FRA.
Succinctly stated in the wikipost. Changes may be added to the wikipost by any signed in member.