Translation:This salad has only tomato and cucumber.
The original sentence is treating "tomato" and "cucumber" as uncountable nouns (like water, milk, bread, etc.), so the mods are expecting the English equivalents to be uncountable as well. Otherwise, if the original said "...τομάτες και αγγούρια", you could say "...tomatoes and cucumbers". Or likewise, if it said "...μία τομάτα και ένα αγγούρι", you could say "...a tomato and a cucumber". Hope that clears it up.