It obviously depends on what the startup does! My favorite startup (2003) was building a shared-nothing distributed system. We chose Java for the distributed p

It obviously depends on what the startup does! My favorite startup (2003) was building a shared-nothing distributed system. We chose Java for the distributed part. Messages were formatted in one of the language-independent serialization formats (Avro? Google Protocol Buffers? I don't remember). We wrote a number of distributed algorithms ourselves, e.g. leader election. We wrote our own logic to detect failed nodes, network partitions, etc., and to react to those events. It was a huge amount of effort, and took a long time to get right. Finally, it worked really well. Shortly after completing the system I discovered Erlang, which had battle-tested solutions to many of the problems we solved on our own. That language would have saved us a LOT of time and effort.