tldr: OCaml
> For Jane Street’s technical rank-and-file, particularly the many hired straight out of university, non-compete agreements may be surplus to requirements. A scan of jobs listed by Millennium, a rival fund that has recently clashed with Jane Street in court, shows the strength of the latter’s position in the job market. Millennium wants engineers experienced in c++, Go, Java and Python, languages that are commonly used across finance and tech. OCaml developers, it seems, are Jane Street’s to keep.
If someone worked with OCaml at Jane Street I would just take this as a signal that they are smart enough to quickly learn Go, Python, whatever they need, and will probably be more successful after 6 months than a “Python developer” would be.
It's a tough situation being experienced in <peculiar language for Company A> when you need to ace technical interviews in <mainstream language for Company B>.
Once you have a few years of promotions, it gets even tougher when you need to compete with <mainstream language> senior+ software engineer candidates at the destination company. Maybe <flashy brand name> was enough to land the interview, but experience mismatches and limitations can remain apparent in the interview itself.
The retention factor is *not* that other companies wouldn't want to hire them, but rather that these employees are likely to dislike being forced to use something other than OCaml.