Re:invent Coming in 2021, Babelfish is an AWS project which provides an endpoint for PostgreSQL that is compatible with Microsoft SQL Server, the idea being to enable porting an application without having to change the code.
The thinking behind Babelfish is that organisations have applications using SQL Server databases that they would love to migrate to PostgreSQL, but that the effort and risk involved in porting is such that they are locked in.
Why is AWS providing this? The idea is to attract new customers either to PostgreSQL-compatible Aurora, or to its PostgreSQL RDS (Relational Database Service).