From what i´ve studied you can use any of the hard good old ones, like C/C++ where EOS is being written itself, as you can see for yourself here:

https://steemit.com/eos/@dan/eos-developer-s-log-stardate-20176-30

This is from another post:

https://steemit.com/eos/@robrigo/consensus-day-1-eos-flyer-the-blockchain-operating-system-for-commercial-scale-decentralized-applications

Scripting Language Independent
The EOS architecture can simultaneously support multiple programming languages. Initially EOS will support Wren and Web Assembly, but it could also support any other suitably high performance sandboxed language.