Translation:Apples are delicious.
Korean, like many languages, doesn't require nouns to be marked for number. This article http://nojeokhill.koreanconsulting.com/2013/01/korean-translation-tip-korea-has-a-plural-form-it-just-doesnt-get-used-much.html suggests that you only need 들 if you want to emphasize that there are more than one of something. Personally, I think that makes more sense.
For 사과는 맛있습니다. English speaker here:
Correct translations are "An apple is delicious"; "The apple is delicious"; or "Apples are delicious."
For absolute plural when you are talking about specific apples, not like that generalized sentence, use 사과들 with the appropriate particle for the words function as subject, topic, or object within the sentence.
bread is an uncountable noun, you cannot use "a" or a number with it, and it doesn't usually have a plural form. you would instead say a slice of bread or a loaf of bread. and if you do say "breads", you would mean types of bread (plain white, whole grain, sesame-seed, etc.)
apple, on the other hand, is countable. you need an article (an, the, this, my, your, etc.) to be grammatically correct, unless it is in its plural form. you then might need or not an article depending on how specific you want to express.