as i know the interviews are conducted in English. sometimes if there are lots of eligible candidates, not being able to communicate in English in a reasonable level might get u off the list, but if u hav a general ability in expressing ur self in English that will suffice.
normally the interview panel will ask general things about urself, then somethings about few current affairs mostly general knowledge, sometimes they might ask how u feel about things that are related to the military. like how do u feel about how we defeated the LTTE or wat is ur opinion about prabhakaran, u know things like that. so if u are ok with talking about anything that some one throws at u u'll do fine.
with the exams im not that sure but, they also might include general knowledge and things pertaining to military and sri lanka, which a general civilian will know.
in all, just speak the truth, and don't think seriously when answering. look them in the eye, address them Sir or Madam. say lots of thank you's hav a small smile on ur face (don't look scared or over excited). u'll do great.
hope this helps.