Oynprith
For centuries, only the vast distances separating Droyne worlds from each other concealed the fact that Droyne worlds had a common language. That is not to say every Droyne spoke the same language; indeed, different Droyne communities spoke different languages, often very different languages.
But all Droyne communities shared one common language — Oynprith, the language used in the coyn casting ceremony. Outsiders missed Oynprith simply because it was not used in public very often; it had a status as a ritual or ceremonial language, much like the status of Latin on Terra.
In interstellar commerce, Oynprith was soon discovered by Droyne to be a universal language among their race; as trade increased, Oynprith became more commonly used, finally reaching a status as the universal Droyne language.