The fact is adapting a service to provide and support a generic API for the long term that others can hook into is extra work, compared to a private API tailored to their own hardware and that they can change whenever they like. It may be they could provide this as an open service in future.

On the other hand, what is to prevent another ear bud manufacturer writing an iPhone app their ear buds connects to that provides translation? Is this really a hardware feature in the phone? If it’s just software at the phone end, as long as other manufacturers have the feature access to implement this themselves, surely that’s their problem? Why should apple offer translation software as a service to other companies for free? I can see the argument for hardware but not software that others could implement themselves on iPhones.