"The words for orange allude to the eastern origin of the fruit and can be translated literally as 'apple from China,' Some examples are German 'Apfelsine' (alternative name for Orange and common in northern Germany), Dutch 'appelsien' and 'sinaasappel,' Swedish 'apelsin,' and Norwegian 'appelsin.'"
Swedish has two genders, neuter and common gender. English a/an translates into en or ett depending on the gender of the word. Gender is more or less unpredictable and basically has to be learned along with each word. Read more about it in the tips and notes section, or have a look in the sticky post under Swedish discussions.