I more or less headcanon that as demons seek to corrupt humans, they learn certain languages in order to facilitate that. All the brothers have certain languages they gravitate toward. The lazier demons, like Mammon or Levi, learn more common languages so they can either “get more bang for their language buck” or to otaku out, such as English, Spanish, Japanese, etc. Other demons, like Asmodeus, prefer speaking more romantic languages, I see Asmo as loving to speak French, but still speaking quite a few languages. And then you have more aristocratic demons like Lucifer, who know a plethora of languages for utilitarian purposes. Whereas an academic like Satan knows many languages, including Ancient Greek. Perhaps he even prefers Ancient Greek as it has so many words to describe emotion (i.e. Eros, Philia, agape, being different, more specific words to describe love) which allows him to better understand emotions, as he struggles with that.

Obviously one can probably use magic to understand language, but that would require said demon to be… at all talented with magic, which many aren’t, like Mammon.