@Getfresh: Thanks for your reply
The main translation was actually all done by the translator(s) but a few sentences (totalling about 30 seconds, but quite key sentences as well) were taken from the translation. Thank you for the tip about 'Additional translations:" it sounds a lot more formal than what we ended up using
'With (additional) thanks to -----'s translation of the ----- novel'.
So I think we will definitely consider using your version next time! Thanks!