ESS-Julia part has a mode called ess-julia-mode. It is derived from julia-mode. This seems from Julia. ESS-Julia support completing objects with company-ess-julia-objects for company-mode. Emacs has default completion support with completion-at-point-functions. It can be used by company-mode with backend company-capf. I hope julia-mode can complete Julia keywords with a function which add to completion-at-point-functions.
Ref https://github.com/JuliaLang/julia/issues/13799