Your oxide/lang files were edited wrongly. Make sure they have no { and } too in the welcome message.

Same issue here. The orignal config has { and } in it:

{
"WelcomeMessage": "Welcome to uMod\r\nThere're currently {0} players online",
"JoinMessage": "Player {0} has joined the server from {1}",
"JoinMessageUnknown": "Player {0} has joined the server",
"LeaveMessage": "Player {0} has left the server. Reason {1}"
}

And so does mine. The problem is still there.