Q: Sent text messages are turning into Chinese

Mine is by length.  Has nothing to do with the type of characters. 

That is correct.  All text no characters of any sort.  No brackets.  Just letters and spaces.

I also have this same issue and I have NOT upgraded to iOS 10. I'm running iOS 9.3.5 on my iPhone 6 and twice now to 2 different people (with non-iPhones) my texts have converted to chinese. Both contain an emoji and were longer than 2 lines. I also have AT&T. I tested the same message without an emoji and it went through.

This is a character encoding issue that currently seems specific to AT&T!  I recently wrote a detailed post about this issue, what's causing it, and what to do about it.  You can read it here:

http://poetryproseandpoppycock.blogspot.com/2016/10/solved-mark-and-mystery-of-o verly.html.

I tend to write in a humorous, yet informative style because tech-talk can get way too dry.  Enjoy the fix!

Glad you found the blog post helpful   It's crazy what an extra binary 1 can do isn't it?

It does not happen with all of my devices just when I text long messages from my MacBook. It doesn't happen in iMessage just when I use SMS. I've not tried to text from my iPad so I don't know if its affected or not. I hope it gets fixed soon.

Interesting!  An iPhone software issue that apparently only affects AT&T.  Or perhaps there are reports of other carriers too?

I still have iOS 9 on my iPhone… and I still have Yosemite on my Mac. When I send the text from my Mac or iPhone to certain Android people, it get garbaged… it's been happening only the last week or two. It's not because any Apple update, because I have not updated in months/years.

YYou are on ATT, right?

yes