Passing language=british to biblatex-chicago with polyglossia

You can make biblatex-chicago into thinking that the language is indeed british:

\begin{filecontents}{\jobname.bib}
@online{key1,
   title = {Stack Exchange},
   url = {http://www.stackexchange.com},
   urldate = {2014-09-16}
}
\end{filecontents}
\documentclass{book}
\usepackage{fontspec}
\usepackage{filecontents}
\usepackage[style=british]{csquotes}
\usepackage{polyglossia}
\setdefaultlanguage[variant=british]{english}

\let\keptlanguagename\languagename
\def\languagename{british}
\usepackage[notes,language=british]{biblatex-chicago}
\let\languagename\keptlanguagename

\setmainfont[Ligatures=TeX]{Cardo}
\addbibresource{\jobname.bib}

\begin{document}

See date formatting in footnote below.\autocite{key1}

\end{document}

Proper support of polyglossia would require changing biblatex-chicago.sty.