That entirely depends on the company you are applying to. Full-stack does not and cannot mean every language under the sun.
Instead, full-stack will mean what the company you are applying for needs it to mean. Is their product a Node/React application with MongoDB? Then you'll need to learn those frameworks and JavaScript really well.
Is their product built on PHP with a templating language for the frontend like Vue.js? Then learn that.
Basically, find out what stack different companies use, choose the one you like, and become sufficient in it. Then you can market yourself as a full-stack developer in that particular arena.
Because of the nuances of the term "full-stack", I've begun avoiding that title as a go-to choice — instead, I lean toward "software developer", which I think implies full-stack, but doesn't necessarily give the impression that I know everything.
In the end, do your best, and pick a specialty.