I posted earlier saying that I thought the update to 8.0.2 had caused the problem to go away (or, more accurately, allowed the language-change fix to work permanently), but of course a couple days after the update it quit working again. I also tried disabling the "Reduce Motion" setting in Accessibility options, and that also seemed to make the language-change fix take hold for a longer period (a couple days), but then it quit working again. And usually, when I do the language switch, it works for only a few minutes up to hours, and then quits. Right now I'm trying a new twist - doing the language switch and getting it working, then doing a reset of the iPhone (by holding the home & sleep/wake buttons for 10 seconds). Several hours after restarting, so far, it's still working. Not holding my breath though.
Slim37, thanks for talking to Apple about this; I really hope they get it. I haven't seen one iPhone user that isn't affected by this problem, so I am amazed they hadn't heard about it earlier. Maybe only very few people use the message search feature, so most iPhone users are unaware or don't care? I can't function well without it, so I hope they fix it soon!