Perhaps using Task Comment could resolve the issue? It's a global comment for a file and it is visible from Mantra.

but how do you enter this as an instruction? this is only available in the review format

I do not really agree. You would then need to do that for every target language (and probably multiple tasks, since you cannot be 100% sure what task the reviewer opens first) whereas the instruction is already available in the job description. I would assume that reviewers can download reference material during their review? If yes, we could add a short instruction as reference material. If not, I feel making reference material available for reviewers is something that needs to be supported in a future update.

Reference files are already available to users via the Client Review interface, so I would agree with Bernd's suggestion on how to approach this.

MATE or similar broker could automatically add job description as a task comment if it's not used for other purpose, but probably that'd be an overkill if reference materials could do the trick.

I guess this would be the current solution. But unfortunately from my experience this is not how Client Reviewers work. They're often people who have other work, their primary job, and review translations on the side. I see that they do not familiarize themselves sufficiently and do not download reference files. Or they only use Mantra so little that they forget functionality. So instead they log in and go right into the review interface and want to get the job done. I believe it would be a much better UX if instructions and background information could be communicated with client reviewers there, right at the top.