"L'éléphant mange des pommes de terre."
Translation:The elephant eats potatoes.
It's a 16th century idiom used by the French to describe this strange new crop returned from South America by Sir Walter Raleigh. La pomme de terre literally means, "the apple of the earth'. Quite poetic really.
It entered common usage and that was enough for it to become the official noun.
"he/she eats" is a present tense conjugation of the verb 'to eat'. It is not a 'plural'.
"He eats a banana." = One person is eating one banana. There are no plurals in this sentence.
"He eats some bananas." = One person eats some bananas. There is a plural in this sentence but it is not 'eats'.
Likewise, 'il/elle mange' is a present tense conjugation of manger (to eat).
There is no fixation with the word 'some'. Different languages have different rules and cannot always be translated word-for-word or literally.