I\'ve recently discovered the awesome iOS5 custom Settings URL Scheme, which can be explained in detail at this great website.
I\'ve found this to work, dir
In iOS 9 it works again!
To open Settings > General > Keyboard, I use:
prefs:root=General&path=Keyboard
Moreover, it is possible to go farther to Keyboards:
prefs:root=General&path=Keyboard/KEYBOARDS