The in-line translations come from Google Translate. People often dismiss Google Translate but it's very good between English and the other major European languages. The quality isn't as good for less popular languages, or if one of your languages isn't English.

If you aren't satisfied with the translation, you can refer to another dictionary in the reader sidebar (or a popup window in the Chrome extension). The default for Spanish-English is WordReference, but users can customize this if they have another favorite (http://blog.readlang.com/2013/11/07/custom-dictionaries.html).

On top of that, you can edit all the words you translate, and if in future you click the same word, you'll get your personally edited version instead of Google Translate. So you're effectively building up your own large translation dictionary. One idea for the future is to use all these edits to crowd source better translations, but there's plenty of lower hanging fruit to get through first!