Localized date with biblatex and polyglossia

Question

I cannot get localized dates in the bibliography when using polyglossia and biblatex. Biblatex's manual says:

biblatex automatically adjusts to the main document language if babel or polyglossia is loaded

However, the following document does not produce the desired output.

%!TEX program = lualatex
\documentclass{article}

\usepackage{filecontents}
\begin{filecontents}{main.bib}
@Article{reference,
  author        = {Doe, John},
  title         = {Some title},
  journaltitle  = {Awesome Journal},
  volume        = {12},
  number        = {8},
  pages         = {1234-5678},
  date          = {2018-12-11},
}
\end{filecontents}

\usepackage{csquotes}

\usepackage{polyglossia}
\setmainlanguage[variant=british]{english}

\usepackage[language=auto]{biblatex}
\addbibresource{main.bib}

\begin{document}
Citation~\cite{reference}.

\printbibliography
\end{document}

The reference is typeset as

John Doe. “Some title”. In: Awesome Journal 12.8 (Dec. 11, 2018), pp. 1234–5678.

While I want the date as

11 Dec. 2018

What am I missing?

Answer 1

As mentioned in How do we get Polyglossia language variants to work with Biblatex bibliographies?, biblatex's polyglossia interface has always been a bit shaky, in particular the detection of language variants does not work. The technical reasons for that are outlined in \DeclareLanguageMappingSuffix, inheritance, and polyglossia in biblatex. With babel language dialects work as expected.

If there is no good reason why you would need polyglossia, I strongly recommend you use babel instead. While polyglossia was intended as a replacement for babel for LuaLaTeX and XeLaTeX and has been recommended over babel for those engines in the past, I believe that the current state of the development of the packages, namely that babel is actively being developed and new features for non-European scripts are added whereas polyglossia development has been relatively slow in the past years, has made that advice obsolete. There are a few areas where polyglossia still has an edge on babel, especially with respect to non-European scripts, but that edge is slowly getting smaller. If you are only planning to write in Western European languages, there is absolutely no reason to prefer polyglossia over babel, see for example this recent comment on the texhax mailing list. See also Decide between Polyglossia and Babel for LuaLaTeX in 2019, Decide between Polyglossia and Babel for XeLaTeX in 2019.

If you must use polyglossia there is a workaround, which is explicitly not recommended. You can remap the english localisation to british with

\DeclareLanguageMapping{english}{british}

To quote the biblatex documentation, p. 303

Note that \DeclareLanguageMapping is not intended to handle language variants (e.g., American English vs. British English) or babel/polyglossia language aliases (e.g., USenglish vs. american).