This thread on the LoL forums by a "Core Team" person seems to answer this:
http://forums.na.leagueoflegends.com/board/showthread.php?t=16318
Relevant excerpt:
Lima Beans - Core Engineer:
we use at least the following languages here
C++ (the core game is written in this)
Lua (core game)
C# (game tools)
ActionScript (game hud and pvp.net)
Java (platform servers)
Erlang (platform servers)
Php/sql (web team / platform)