Full interpretation has to be provided. It is highly sensitive stuff, with issues of cultural diversity, national pride and democratic legitimacy always coming up against hardheaded officials worrying about budgets, efficiency and logistics. Yet there is no choice. For him the EU isn't multilingual at all. It just speaks to him in his language.
Interpretation is technically more complicated.
With 11,000 meetings a year and 50 to 60 every day, there are already 110 possible combinations when working with the current 11 languages. With 25 languages the figure will rise to 600 and the chances of finding someone who can turn Greek or Portuguese into Slovak or Hungarian are virtually nil - even though there are some weird and wonderful combinations of expertise. But ministers meeting in the council have to have their own interpreters - thus that awkward silence over the Basque problem. But overall it is a tried and tested method. Relay interpretation creates practical problems too, not least of finding the physical space for all those booths.