I remember when I did my military "oath" - it just was sufficient to stand in formation and repeat the words - it did not matter if you did so loud and clear or if you were dead silent (like me) - as long as you were pressent in formation on that occasion, that was it - done - the only time I did wear the Großen Dienstanzug as well, ever.
I was a conscript however full professionals may have had to sign a document.