The "aichat" message is not a network message, so this case cannot occur / dead code.

Appears like this commit made the "chat" simulation command unused (and obsolete because there is no use case for a simulation sent network message that is not translated, unless it was considered an appropriate format for player chatlogs.
But we don't, because the simulation messages are not processed until the next turn, so chatting during pausing wouldn't be possible with that format. (The AI in fact has the same problem that it can't pause during simulation pause, but I guess chatting with AI during pause isn't a wanted feature.)

As it stands, the feature also allows players to send simulation chat messages (with those ending up in the replay but not the regular chat messages), which is probably not designed feature but a side effect.
The AI could send notifications to the GUIInterface through the engine instead of making it a simulation command available to all players.

Then both "chat" and "aichat" simulation commands can be removed.