Like I've said many times before, the word "jeruk" can be both singular and plural.
From my understanding, almost all of the nouns, perhaps except pronouns. If u want to be specific you say things like "dua buah jeruk" (meaning "two oranges", buah is a classifier) or "seekor ikan" (meaning "one fish", ekor is a classifier).
Indonesian nouns doesn't differ between singular and plural form. If you want plural noun you can repeat the noun (but not when the amount of the noun is stated directly).
Orange = Jeruk Oranges = Jeruk-jeruk Two oranges = Dua jeruk
Hope this helps.
They should add "sebuah" before "jeruk", so people can guess whether it is a singular or plural noun.