"My prince" is not a direct translation for "Knyaz"
-Firstly, it is a Slavic title found in most Slavic languages, It is usually translated into English either as Prince or less commonly as Duke, however, both terms do not represent a sufficiently accurate translation, because Knyaz was derived from the Proto-Germanic kuningaz (king).
-Secondly, "Knyazhe" is a vocative, which has almost disappeared from modern Russian, and which can't be translated to English though it has an essential pattern of meaning that refers to fairytales and very old times.

Erika_hermi, hope this translation helped you as well

Advices and corrections are very welcome