If you want a complete translated text, I suppose you should include the originally English stuff also. In some cases you may want to mark it as [English original] or whatever if you think the reader should know that. You'll get a few points for translating when you really didn't do anything, but we all need a rest once in a while. If you feel bad about that, you can translate the English into Spanish!

If no one "translates" those sentences, the article will be considered "incomplete" forever, so it's helpful in that way. A solution that Duolingo could implement is a checkmark for "this sentence is already in English" that people could click for 1 point or something.

Thanks for the comments. They clear up the matter for me.