This is not an acceptable English sentence. Aside from the fact that the plural of fruit is usually fruit, the verb in an appositional sentence (x is/are y) always agrees in number with the subject. Today's snack is... / today's snacks are..., whether or not the noun in apposition agrees in number. If we drop the unusual "fruits" the only translation that will not identify you as a non-native speaker is: "Today's snack is fruit." The fact that no particular fruit is named implies that there will be a choice.
"the snack today is fruit" should be accepted - it might not be 100% proper based on fancy-shmancy books, but that's what people say in SD, CA, and allover america. -native speaker for 34 years. : )
there are other ways to translate the sentence properly, but "the snack today is fruit" should also be accepted.
English speakers would never say "Today's snack are fruit." Snack is singular not plural.
What about : "The snack is fruit today" ? I was taught that time words are at the end of a sentence.
A tricky one! "Today's snackS (plural) ARE fruit" "Today's snack (singular) IS fruit" The plural of "fruit" is "fruit", but the result of growing many different kinds of fruit will be "fruits" of that labour.
I really don't get the singular/plural thing in Portuguese. For example, "eu vou pagar para a uva" is all about paying for the grapes, whereas this sentence uses "sao frutas" when it is about fruit. OK, it is Portuguese and not English, I get that. But, how do I know when something is meant to be singular and when it is meant to be plural?
Shouldn't it be "O launch de hoje é fruitas." Is it good Portuguese to say that any singular subject should be followed by a plural form of the verb?
The verb "ser" is a very particular case.... There are many rules related to it.
Here, the best choise is "são" because the predicative is plural.
Please, see letter "b" in bullet list here: https://www.soportugues.com.br/secoes/sint/sint56.php