Programming languages are like tools in a toolbox. The one you choose depends on the job you are trying to accomplish, and where you are doing it.
Responses from Fred, Jerome, and Satya about cover it. You can look up job description for Full Stack Developer on sites such as indeed.com to see the skill set range. Indeed link: https://www.indeed.com/jobs?q=full%20stack%20developer&l=San%20Jose%2C%20CA&start=10&vjk=5ce7896bec67e9a5
Snip from a Apple listing:
Expertise in at least one scripting language such as Python, Perl, or TCL.
Experience with RESTful API usage and development
Good understanding of at least one web framework (Django, Flask, Rails, etc)
Experience with version control (i.e. git, svn)
SQL, no-SQL database experience
Test driven development
ADDITIONAL DESIRABLE QUALIFICATIONS
Application container deployment with Kubernetes, Docker
CI/CD tools - GitLab, TeamCity
Data structures and algorithms
Knowledge or interest in learning CPU architecture and debug
That said, the MERN (MongoDB, Express, React, and Node) stack is popular these days - toss some SQL in there.
Consider that you have a latency in the system and are thinking you should use redis to solve this. You should first know what Redis offers and what are the other options to make it work. The requirement specifically determines the need to use a particular language or a tool. Like Pieces in a Puzzle, only the right piece (Language or Tool) will fit the requirement. So, you should be open to learn and use anything.