What a shame, i'm german and i've read lots of submarine-books. (just finished "Das Boot" again)
As far as i can tell, the crewman confirms the fired torpedo by saying "ist los" (is out / running) or "im Wasser" (in the water). Or as Catfish said "Torpedo läuft" (torpedo running) - It's definetly not "gefeuert" (fired).
But, what i know, regarding guns, "Feuer frei" is just an expression for "start fire" or "fire at will", and doesn't mean "choose any target and fire at will". Usually the targets are advised by any officer.
I'll check again by my german version.