The interview is conducted in English, even for posts based in non-English speakers countries. Questions are behavioral and it is very important to read about the British Council Behaviors so you can prepare yourself to give clear examples of your qualifications and profile. The interview take around 45' if you are well-prepared and they often make follow up questions if you were not clear, so take every opportunity to make a strong statement about yourself. And DO ask questions at the end, it usually stands out!