Comparative law notes
For the most accurate translation the phrase "übereinstimmender Parteiwille", one would need to write out the whole phrase in English, "mutual assent of the parties".
übereinstimmender Parteiwille
Substantiv
full
mutual assent
noun
Definitions of mutual assent
agreement to enter into a contract by each party to the contract through offer and acceptance
The parties' words and/or acts indicate mutual assent.