And is the language in cockpit one that the BEA head would understand? In an emergency situation would you revert to your language of birth, aviation English or a mixture. Surely as a professional investigator he would wait for a native speaker to interpret the language used and its idiosyncrasies.
The german BFU has three investigators in france to take part in the investigation. I guess one pretty big part will be the voice recordings. I guess the official flightdeck languages at lufthansa are german and english, with all the checklist and procedures in english, the more relaxed personal talk in german. At least that is how the german competitor handles it. ATC of course is always in english.