A really big input box. Because why not.
How to Use
You just hit Enter and type something. If you wish to move it, hold down a modifier like Shift, and drag it by its label/title area. To reset the position, hold down Shift, and click its label, but not drag. Tadaa!
How to change character language
Just hover the cursor above the box, and click the language text appearing right below it to toggle your character's spoken language. When a non-default language is chosen, the language text will remain visible as long as the editbox is visible, reminding you of what language you are typing in.
Why doesn't the keybind for opening the chat slash work?
Blizzard limitation. That would require our addon to be responsible for having put the / character into the editbox, which in turn would be considered a change to the secure editbox execution path and finally result in you NOT being able to cast spells or perform secure actions through chat.
This is a part of Blizzard's anti-bot systems, as the ability for addons to do this would ultimately allows us to create bots that could cast spells through addons.