'Naha' means 'No' in Sinhalese. If you don't understand what someone says to you, just say 'mata therenne naha', so that the speaker will either tell that to you in English or try to slow down a little bit. If someone keeps on talking to you in Sinhalese, you may kindly ask 'oyata English katha karanna puluwanda?' which means 'Can you speak in English?' This will be useful for you especially at shops and markets.