Em regras da ICAO , se a Alemanha 'validar' a sua licença italiana, então sim, você pode voar a aeronave alemã em outros países:
Use of flight crew licences on foreign-registered aircraft
Any pilot who wishes to fly on an aircraft registered in a State other than the one that has issued the licence, needs to obtain an authorization from the State of Registry. This authorization is generally given by the State of Registry through a validation or a conversion of the foreign licence. In general, the validation process is used for short-term authorization while the conversion process is used for longer-term authorization.
O que significa "validação" aqui é o país onde a aeronave está registrada. A ICAO em si não define o que deve fazer:
When a State validates a foreign licence, it recognizes it as valid for use on aircraft on its own registry. The Convention on International Civil Aviation and its Annex 1 do not contain specific requirements for the validation of licences beyond establishing the principle and the fact that the validity of a validation, cannot be extended beyond the validity of the supporting licence.
Nos EUA especificamente, essas regras da ICAO são implementadas em 14 CFR 61.3 (ênfase aumentada):
(b) Required pilot certificate for operating a foreign-registered aircraft within the United States. No person may serve as a required pilot flight crewmember of a civil aircraft of foreign registry within the United States, unless—
(1) That person's pilot certificate or document issued under §61.29(e) is in that person's physical possession or readily accessible in the aircraft when exercising the privileges of that pilot certificate; and
(2) Has been issued in accordance with this part, or has been issued or validated by the country in which the aircraft is registered.
Na prática, eu diria que, se você tiver algum documento oficial das autoridades alemãs confirmando que você está autorizado a operar aeronaves alemãs - ou pelo menos esta aeronave alemã - então você está pronto para ir. O pedaço de papel pode até ser desnecessário no que diz respeito à Alemanha, mas se você planeja voar internacionalmente, então eu acho que quanto mais pedaços de papel oficial você tiver, melhor.