There are two other strings with "Changed Their Avatar". One indicates it is for the feed. Try changing those as well. I did a search for "changed their avatar" in "all sections"
See the screen shot.

thanks Darryl B!thank you for your answer. but i tried, it failed too 

Try clearing your browser cache. It may be loading cached info.

I tried it on my test site, and didn't see the change either. It may be that it will show the next time someone changes their avatar. You could try setting it to what you want then change your avatar to see if it shows then.

Yep. That's what it is. It won't change ones that are already displaying. It did show when I changed mine with a change in the three language sections.

thank you! but i found " join the site" is changed when you change the info. the code of " join the site" is the same with "changed their avatar" is it a error? 

Shouldn't be the same. If I'm understanding correctly, when you change the "Changed their avatar"; it also changes "Joined our site"?

ok i will test it ,thanks Darryl ~　thank you for your help 

You're welcome. After you make the changes, you should see them with new feed posts. Any before the change will show the old language key. They may, or may not change once the cron runs.

ok it is ok !thanks