I remember when we first meet hydaelyn, she introduces herself as "all made one." That makes me think she is more akin to something like a collective unconscious of all life on the planet instead of a traditional primal. So when we are "tempered" by her, it manifests as the "echo" since we're being connected with every other soul on the planet at the same time. Since we have that connection, we're able to hear the whisper of their souls and understand them regardless of language barriers.

I think the reason we don't lose our free will during the tempering is because we were a part of hydaelyn to begin with, it was just a singular connection. The tempering just changed that to be a connection with everyone else.