documentclass{article}
usepackage{polyglossia,fontspec}
setdefaultlanguage{english}
setotherlanguage{latin}
setmainfont{Source Sans Pro}
newfontfamilySmbd{Source Sans Pro Semibold}

begin{document}

1. normal weight  textit{italic} textlatin{foreign} blah

2. textbf{bold   textit{italic} textlatin{foreign} blah}

3. {Smbd{semibold textit{italic} textlatin{foreign} blah}}

end{document}