In my opinion....Not much tbh!
To make a stable server you know to know basic programming languages such :
MySQL ( SQL Server )
PHP ( Website Part , to connect web with your db)
Assembly x86 (to understand your crash logs by checking the offsets logs provide you)
That's is about programing languages to make your own basic server , but this change if you want to make an advanced Shaiya Server, you should mainly do C++ ( shaiya is build on C++) but as i know Python,Java works on shaiya too ( there are cool Dev that works on Java or Python )