documentclass[a4paper, 12pt, english]{report}
usepackage[rm, light]{roboto} %this makes roboto slab light the main document font
usepackage[T1]{fontenc}
usepackage[utf8]{inputenc}

usepackage{titling}
titleformat*{section}{sffamilyLarge} %with sffamily I try to set roboto regular as section title font

begin{document}
section{I need to be in roboto font}
I am normal text and need to be in roboto slab light font.
textit{I am in italics and need a different font, e.g. roboto}
end{document}