Hello,
I understand your concern about the watermark[0] observed on your translated Doc. This is an attribution requirement[1][2] and sadly, it cannot be removed now.
The Translation API product team is aware about the concern and they are surely looking to make the necessary adjustments but there is no ETA for that.
I hope this information helps.
[0]“Machine Translated by Google.”
[1]https://cloud.google.com/translate/attribution?csw=1
[2]https://support.google.com/translate/thread/115298680/how-to-remove-machine-translated-by-google