If you speak in German with one contact, Emoji with another, and Arabic with yet another, Messages will restore those keyboards each time you switch back to those conversations.
This is very handy for folks who write in different languages, and one of those wonderful, thoughtful little touches that makes Apple’s products extra special.
To add and remove different languages, including Emoji, go to Settings > General > Keyboard > Keyboards > Add New Keyboard.
[thanks @deanishe!]