In the first half of the first episode of Hataraku Maou-sama, we can hear conversation between Satan and Alsiel in "Underworld" language. Is the "Underworld" language based on a real language, or is it just plain gibberish?
According to this article (which cites from a 2ch thread), the language is is pretty simple:
The Entean language is basically letter substituted English, with vowel keeping their original position, while the consonants are reversed, so B=Z, C=Y, D=X, ... and vice versa.
ABCDEFGHIJKLMNOPQRSTUVWXYZ -> AZYXEWVTISRLPNOMQKJHUGFDCB
- 人間 human → tupan
- 世界 world → foklx
- 魔王（芦屋による呼称） majesty → pasejhc
They say it with such enthusiasm, too:
Based on this thread from MAL:
It seems that it IS gibberish. They just jumbled a lot of languages together (English, French and German to name a few).
Here are some quotes from the thread (I am still not sure if they can be counted on as being 'facts'):
The way they threw random words of other actual languages into their made up language was very funny, especially if you understand Japanese (how foreign words are transliterated/pronounced in a syllabary language). - Usuzumiiro (1)
I tracked these down by just going to page 1 of that thread then searching for the word "language". It seems the general consensus is it is gibberish and that it is a mixture of words. Still looking for the actual source for them all to say that though. (I am thinking it is on 4chan)