I completed front end HTML,CSS, JavaScript.which languages are best for back end in 2019

Try PHP

You have not completed it Try to learn Es6 till I think latest is ES10 or ES 2019 And Try make a portfolio site

For back end you should do the SQL lesson and also take a look at jQuery

C++ actually works really well for this, it’s also great in performance. But you cab also try php or sql.

try SQL :v