If you want to know what Languages are used, try visiting
www.sourceforge.net and check what the more popular projects are using. Here's the data from the 10 most active (I just picked a metric by whim). I must admit that even I am surprised by the breath of Languages used. Assembly was a definite surprise

If you're going by this sample, you should learn PHP and C++
Java II
JavaScript II
PL/SQL I
ASP I
C# I
Cold Fusion I
PHP III
C++ III
C II
Assembly I
Python I
The projects were:
Gaim
Azureus - BitTorrent Client
ABC [Yet Another Bittorrent Client]
phpMyAdmin
eGroupWare: Enterprise Collaboration
PearPC
FileZilla
WinMerge
FCKeditor
Compiere ERP + CRM Business Solution