By EU DMA, Apple is not allowed to develop a free feature in iOS (!) in order to recoup the cost by restricting it to their own brands and crushing the competition with it in ANOTHER product-segment (in this case, Headphones).

They could easily make this live translation feature a separate app using publicly available iOS-APIs. Every competitor would be able to develop and provide the same feature.

Ah, not integrated enough for them? Fair enough, then whatever further integration they see fit needs to provide interoperability for competitors.

That's exactly the stance of the EU DMA.