How many of you were in a situation Matt Groening described?
Those who were and realized the problem, know firsthand, that one can't really know the language without knowing its idioms.
It's easier with our mother tongue, even though it's not always the case either. But it's 100 times harder when it comes to a foreign language.
Well, you might find some interesting stuff on this site. We in no case are going to replace multiple sites that have great collections of idioms. There are a couple of nuances here that you might find interesting.
1. We are going to provide you with explanations in plain language of what various idioms mean.
2. We will be doing cross-references across multiple languages.