... queue broken record.
no platform with billions of accounts can be built using just a single language
WHY?!?
In what way is that a valid statement? Handling billions of accounts is a matter of scalability and load distribution. So long as the proper planning is in place there's no reason you can't just keep adding more servers -- which given the numbers we're talking we are CERTAINLY talking several dozen if not hundreds of servers working together.
Whilst yes, some languages are better suited to certain tasks, there is ZERO legitimate reason you couldn't use just one server-side language to do it.
Sheesh, and I get accused of making "crazy" blanket statements.