Anselm Grün is a prolific author. My understanding of the meaning of the book titled Die Freude wird Vollkommen sein is The Joy Will Be Complete. This book was translated into Spanish under the title La Alegria, Contra el Desconseulo de nuestro Tiempo, which I think means something like Joy against the Dispair of our time. I have looked for Anselm Grün+Joy, Anselm Grün+Happiness, but I cannot figure out what this book translated into English would be published as. Can anyone tell me?
I guess the title alludes to Philippians 2,2, which reads in German:
dann macht meine Freude dadurch vollkommen, dass ...
The English Standard Version has:
complete my joy by ...,
so your translation is fine.
Grün's German publisher Herder translates the title as
Joy will be perfect
(see herder.de/foreign_rights) and informs us that the Hungarian, Italian, Polish, Slovenian, Spanish (AR), Spanish (ES) rights of the book have been sold - no mention of an English translation.