Saw this on my server a long time ago. Wasn't sure if it was a source change or what, but once I had found the issue being related to common tongue skill I didn't look into it beyond a quick hot-fix.
One thing to check - what your chat window default language is.
Two - if Common is less than 100 for anyone you will see issues like this.
Can fix it by putting
Code:
$client->SetLanguageSkill(0, 100);
In EVENT_CONNECT or EVENT_ENTERZONE.