If you are willing to root your device there are at least two solutions in the Google Play store that claim to do what you want: Keyboard Manager and Keyboard Master.
For non-rooted devices, there is Keyboard Manager Plus - however you should be aware that it flashes the keyboard picker on the screen as it automatically changes the keyboard. If you can live with that, that's a possible solution for you without rooting.
Caveat: I have not tried these myself. I am researching the issue and I do not have the option of rooting the device and I cannot live with the flashing. However, I've taken a pretty close look at all three of the above and they are the closest I've found to answering your question.