The word mchumba is a Swahili word and we normally use it in two genders namely feminine and masculine. So a girl can say this is my "mchumba" meaning a boy who is going to marry her in the near future, a husband to be. Likewise a boy can also say this is my mchumba, a wife to be. Therefore, as far as the gender is concerned, the word mchumba is both feminine and masculine and as far as the meaning is concerned the word mchumba means a husband to be not only sweetheart. Of course, one cannot plan to marry someone whom he or she doesn't love hence sweetheart is within the word mchumba.

Actually you can plan to marry someone whom you don't love, so 'sweetheart' is not the same as 'fiance' as you are saying it is.