Here is a merged list of the opcodes values I found and those provided by tom_rus (thanks a lot!). There's also a handful additional ones.
Paste2: Next Generation Pastebin - Viewing Paste 841350
And here's the patch for mangos's 400 branch with all those opcodes in:
Paste2: Next Generation Pastebin - Viewing Paste 841346
I still can't figure out why my client stubbornly says "You cannot speak that language" when trying to say something in chat. I don't know that much about wow server emulation, so I can't figure out what thing the client needs to be sent for it to work.
It seem to work rather well other than that so far but testing is kinda limited without access to gm commands.
I guess that I'm gonna look into spell casting related opcodes instead next.
Also it's a relief that blizz don't seem to randomize the opcodes automatically right in their build scripts =)