Great developers are curious. They are naturally polyglots.
Curiosity leads to inspiration. José Valim, creator of Elixir, read Seven Languages in Seven Weeks and used those languages for inspiration for Elixir. Evan Czaplicki drew heavily on ML for Elm inspiration. Joe Armstrong and the Erlang team based initial versions of Erlang on Prolog, and Robert Virding from the same team has built another inspired language, Lisp-Flavored-Erlang. You don't need to be a language designer. You'll get inspired too.