Adding an extra keyboard button for switching to rarely used languages?

Question

I type in the following 3 languages:

1. Language A - 55% of the time
2. Language B - 40% of the time
3. Language C - 5% of the time

The usual solution would be to have all three languages installed, however it is quite annoying to have to switch over Language C each time I swap back and forth between A and B.

Is there some sort of solution that would allow me to either have an extra button for language C or switch to language C using a special key combination?

I'm using Nexus 4/Android 4.4.

Thank you.