Hi aspirant,
Language used for SPI interview is english itself. So you need to be learn to be able to speak in english. Dont panick on listeining to it. When you will enter the hall there would two or three peesons seated there. Dont have a seat until you are asked. They'll start with simple questions like what you had in your meal or such questions. Then they will ask you questions regarsing your favourite subject or sports. So make sure you are ready with ehat your favourite subject is. The atmosphere is quite friendly so there's no need to worry. Best of luck.