It might be helpful to provide a way to hide the L1 definition as a way of encouraging thinking in the target language where possible.
When I took German at the Goethe-Institut, they were extremely committed to immersion and moderately discouraged the use of bilingual dictionaries, while the teachers would almost always attempt to explain vocabulary in German (or by example, like by acting something out or pointing to it). This is harder if you're very new to a language and don't have a live teacher present, but at some point it probably switches to being a benefit for many or most learners.
The German teachers also encouraged students to try to explain vocabulary (to each other) in German, on one occasion literally having us play the game Taboo (in which you have to get people to guess a word without saying that word or a few closely thematically related terms).
As an example from your site, when I tried it out with EN→PT, I encountered the defined word "joelho" 'knee' (which I already knew), and the definition used "coxa" 'thigh' which I had encountered in the past but mostly forgotten. It was helpful to me to see the Portuguese definition and possibly less helpful to see the English definition (because relying on it would have been a missed opportunity to refresh my knowledge of "coxa"). As an analogy, if I watch a movie with English subtitles, I almost always end up ignoring most of the spoken foreign language even if it's a language that I'm fairly familiar with, because the subtitles just make things too easy.
I definitely think having the L1 definition easily available is important (in particular, in case the user doesn't understand all of the L2 definition!) but I suggest at least allowing people to choose not to see it every time, so that they can practice thinking in the target language.