bym, the "Demonic Language" is central to Demonolatry, as practiced by the now defunct First Temple of Demonolatry and discussed on Tezrian's Vault(also closed). We're not really sure about the origins of this language, however, the interesting thing is that the invocations we use(called "enns") have been the same throughout history around the world. This shows us that there's something else at work since this isn't physically possible. If you've ever heard about the Dukante Hierarchy of demons(a quick google search and possibly looking at caches will provide results), this is the main hierarchy of Demonolatry, and Dukante was a Demonolator in the FToD type of practice(though the Temple didn't exist as an entity, the practice was in existence. It's rooted in the Hermetica which is a pretty old text). Demonolatry is now an underground religion b/c the response really wasn't what was expected(many people asked dumb questions like "do you sleep with demons", etc., sites plagiarized the book Modern Demonolatry, copyright infringement, etc.), so the sites, online stores, etc. were taken offline. It's not a religion you can make up, but one you must learn from a Demonolator. Lineage is very important in practice. However, you can still find caches of the old sites. Here's the info you might find interesting:
http://web.archive.org/web/20001118134200/...nting/gynna.htmhttp://web.archive.org/web/20010303015450/...cium/dukhi.htmlhttp://web.archive.org/web/20010303155443/...m/pantheon.htmlhttp://web.archive.org/web/20010303223258/...icium/wvsd.htmlhttp://web.archive.org/web/20010124053300/...atry.com/crypt/http://web.archive.org/web/20010502201315/...atry/myexp.htmlhttp://web.archive.org/web/20001025183323/...try.org/faq.htmhttp://web.archive.org/web/20010502203755/.../conjuring.htmlOne of those has an example of the demonic language. The more in depth information, invocations, family grimoires, Dukante Grimoires, rites, etc. are not and never were onilne. There's also info on the subject of invocation. Invocation is a part of basically every religion, especially the more "experiential" ones(Hinduism, Catholicism, Demonolatry, Wicca, types of Paganism, etc.). We call the energies of the Demons into our circles(note that the purpose and construction of the Circle in Demonolatry is very different from traditional Wicca, ceremonial magic, etc). The Demons are not evil, malevolent forces to be bound, commanded, etc. They are gods and goddesses, many of which were "demonized" by Christianity during its advent. The set invocations we have in Demonolatry are very powerful(similar to the power of mantras in Hinduism though used slightly differently)and are meant to be used only for invocation, not evocation. Therefore, we never evoke in Demonolatry.
Hope that all made sense!