I've had this reaction a few times, especially since a LifeHacker article (http://lifehacker.com/readlang-helps-you-learn-a-foreign-lan...) which compares it to the Language Immersion extension (https://chrome.google.com/webstore/detail/language-immersion...) which works exactly as you describe.

Readlang is intended to be used in reverse - you should be using it to read Spanish articles, and clicking on the words and phrases you don't understand to translate them to English. Learning from actual native Spanish texts is vastly preferable to machine translated Spanish. Of course the downside is that you need to:

a) have some ability in Spanish already

b) develop a new habit of visiting Spanish websites. You can get an idea from the most popular sites visited by Readlang users here: http://readlang.com/es/links (I've been thinking of exploring another idea for an extension which hijacks the Chrome "new tab" page to encourage this habit)