One of my bottom two front teeth is pushed a little behind the other one, and the edge of the tooth is a bit sharp. Every now and then, my tongue will get irritated from that sharp edge and will hurt; fortunately, that usually goes away after a day or two.
This morning, it's happened again, and I noticed that speaking English will make my tongue hurt, because the way I make the two "th" sounds causes my tongue to flick against the sharp edge, irritating it. That's not a problem when I speak German, for obvious reasons :)
I briefly considered speaking only German, but didn't want to, for Amy's sake (to avoid confusion). I also considered using [f] and [v] instead but didn't think I would be able to keep doing that; I'd slip up often and so I wouldn't have gained much.