Ok, here's my story, hopefully it'll help those of you who are still having problems with this.
When I upgraded to iOS7 last year on my iPhone 5, I had the same problem. This distressed me greatly, because I manage apartment buildings, and I keep track of tenants contact info in my text messages; as long as I use their name somewhere in a text, I can search it to find their phone number, or if I miss a call from a tenant, I can search for the phone number in my messages, and see any texts we exchanged so I can identify who it was. So, when I can't search messages, it makes it a real pain.
I finally found that if I did an iCloud backup, erased the iPhone, and restored from the backup, the message searching worked again. Problem solved.
Now, with 8.0, now on my iPhone 5s, the same thing happened. I said "crap", but I knew what to do - I backed up to iCloud, erased the phone, restored from the backup, and boom, message searching worked.
For a while. Then it stopped. AARGH! I did it again. Same thing - worked for a while (like half a day) then stopped. That's when I tuned into this message thread. I tried switching languages. No luck. I updated to 8.0.2. No dice. Tried switching languages again. No cigar. Then I did the backup-delete-restore again, and finally, it started working, and it has stayed working for a couple days now. Whew!
I never tried switching languages multiple times and waiting and all that rigor that was posted more recently, but it certainly seems that 8.0.2 was the difference in at least getting the backup-delete-restore method to work, so if you're having trouble still, try that.