"Tomatoes are fruits."
Translation:Les tomates sont des fruits.
That is not a correct translation. "Des" is simply the plural of "un" or "une." When you say "tomatoes are fruits" you mean all tomatoes in general are fruit, not just two or three of them. In French, this is always expressed with the definite article. In this case, it's "les."
- Tomatoes are fruits. = Les tomates sont des fruits.
Also note that this only applies to the subject of the sentence, because the general statement you're making is about the subject. "Fruits" still makes use of the indefinite article "des" because you're not making a general statement about fruits.