buycraft error

org.bukkit.command.CommandException: Unhandled exception executing command 'buycraft' in plugin Buycraft v5.4
	at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46)
	at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:189)
	at org.bukkit.craftbukkit.v1_5_R3.CraftServer.dispatchCommand(CraftServer.java:523)
	at net.minecraft.server.v1_5_R3.PlayerConnection.handleCommand(PlayerConnection.java:965)
	at net.minecraft.server.v1_5_R3.PlayerConnection.chat(PlayerConnection.java:883)
	at net.minecraft.server.v1_5_R3.PlayerConnection.a(PlayerConnection.java:840)
	at net.minecraft.server.v1_5_R3.Packet3Chat.handle(Packet3Chat.java:44)
	at net.minecraft.server.v1_5_R3.NetworkManager.b(NetworkManager.java:292)
	at net.minecraft.server.v1_5_R3.PlayerConnection.d(PlayerConnection.java:109)
	at net.minecraft.server.v1_5_R3.ServerConnection.b(SourceFile:35)
	at net.minecraft.server.v1_5_R3.DedicatedServerConnection.b(SourceFile:30)
	at net.minecraft.server.v1_5_R3.MinecraftServer.r(MinecraftServer.java:581)
	at net.minecraft.server.v1_5_R3.DedicatedServer.r(DedicatedServer.java:226)
	at net.minecraft.server.v1_5_R3.MinecraftServer.q(MinecraftServer.java:477)
	at net.minecraft.server.v1_5_R3.MinecraftServer.run(MinecraftServer.java:410)
	at net.minecraft.server.v1_5_R3.ThreadServerApplication.run(SourceFile:573)
Caused by: java.lang.RuntimeException: Language key 'mainCommand' not found in the language.conf file.
	at net.buycraft.util.Language.getString(Language.java:106)
	at net.buycraft.commands.BuycraftCommand.process(BuycraftCommand.java:114)
	at net.buycraft.Plugin.onCommand(Plugin.java:106)
	at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44)
	... 15 more