Can you please send me your lang file? 

Whenever a message is entered in chat the following error is showing in the console.

Failed to call hook 'OnUserChat' on plugin 'DiscordMessages v2.1.0' (FormatException: Index (zero based) must be greater than or equal to zero and less than the size of the argument list.)
  at System.Text.StringBuilder.AppendFormatHelper (System.IFormatProvider provider, System.String format, System.ParamsArray args) [0x000ff] in :0
  at System.String.FormatHelper (System.IFormatProvider provider, System.String format, System.ParamsArray args) [0x00023] in :0
  at System.String.Format (System.String format, System.Object[] args) [0x00020] in :0
  at Oxide.Plugins.DiscordMessages.GetLang (System.String key, System.String id, System.Object[] args) [0x00017] in <1e17d3b6d3f047a8967dc061792cecd8>:0
  at Oxide.Plugins.DiscordMessages.HandleMessage (Oxide.Core.Libraries.Covalence.IPlayer player, System.String playerMessage) [0x000db] in <1e17d3b6d3f047a8967dc061792cecd8>:0
  at Oxide.Plugins.DiscordMessages.OnUserChat (Oxide.Core.Libraries.Covalence.IPlayer player, System.String message) [0x00011] in <1e17d3b6d3f047a8967dc061792cecd8>:0
  at Oxide.Plugins.DiscordMessages.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x0015a] in <1e17d3b6d3f047a8967dc061792cecd8>:0
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <9affce1cd15c4ec183941adef8db1722>:0
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <4452f821def6406d834e4149849fe7ea>:0
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <4452f821def6406d834e4149849fe7ea>:0

I fixed this by deleting all files related to DiscordMessages and reinstalling and inserting my webhook links back into the config.