I am not an expert but you might use this ? 

Thanks for the input, and yes that rule will only apply to the English language pages, but that "Audience" created on contoso.co.uk/episerver/cms will still be visible to e.g. German editors logging into contoso.de/episerver/cms
Ideally we would like to avoid German editors to avoid seeing the Audiences of the English editors.

So it seems you want to have an access right settings for Audience also?

That could be one solution. An optimal solution would be specifying what websites each Audience can be used in, so they aren't confusing and visible to use elsewhere.

good idea. I created a suggestion to our CMS team. it will be a long way to be materialized, but we'll see 

Not exactly what you're looking for, but here is an example of making Audiences (Visitor Groups) site-specific: 

https://kunalshetye.com/posts/customizing-visitorgroup-behavior-in-optimizely-cms/

Could be a starting point to extend it further?