# How to cite an item with a parallel Ancient Greek title?

 \usepackage[utf8]{inputenc}
\usepackage[LGR,T1]{fontenc},
\usepackage[polutonikogreek,english]{babel}
\usepackage[backend=biber]{biblatex}
\usepackage{filecontents}

\begin{filecontents}{myfile.bib}

@Book{TH,
title = {The theory of Havens = Θεορια τον μετεωρον},
language = {english}, %then error. -> \textgreek{Θεορια τον μετεωρον}?
}

\end{filecontents}

\begin{document}

Test.\cite{TH}

\end{document}

• There is the field origtitle, but the manual says it is not used by the standard styles, so you have to add support for it. – egreg Feb 11 at 18:07
• What is the exact output you are looking for for the bibliography entry? – David Purton Feb 11 at 23:31
• Just "The theory of Havens = Θεορια τον μετεωρον" or The theory of Havens (Θεορια τον μετεωρον). – Dmitry Starostin Feb 12 at 16:07

The simple approach of writing

title = {The theory of Havens = Θεορια τον μετεωρον},


fails for exactly the same reason that just writing

The theory of Havens = Θεορια τον μετεωρον


in your document fails: The language (and in particular the font encoding) is not switched properly.

The easy way out is to switch the language explicitly (like you would in the document).

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[LGR,T1]{fontenc}
\usepackage[polutonikogreek,english]{babel}
\usepackage[backend=biber]{biblatex}

\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@Book{th,
title = {The theory of Havens = \foreignlanguage{polutonikogreek}{Θεορια τον μετεωρον}},
}
\end{filecontents}

\begin{document}
Lorem \autocite{th}

\printbibliography
\end{document}


If you are also looking for a way to make the input more semantic, you could try the origtitle field as suggested by egreg in the comments. Note that for most intents and purposes the orig... field have been superseded by the related features, but here it might make sense. The standard styles don't print that field by default, so we have to modify a few things to get it to work.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[LGR,T1]{fontenc}
\usepackage[polutonikogreek,english]{babel}
\usepackage[backend=biber]{biblatex}

\renewbibmacro*{title}{%
\ifboolexpr{
test {\iffieldundef{title}}
and
test {\iffieldundef{subtitle}}
}
{}
{\printtext[title]{%
\printfield[titlecase]{title}%
\setunit{\subtitlepunct}%
\printfield[titlecase]{subtitle}}%
\newunit}%
\printfield{origtitle}%
\newunit

\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@book{th,
title     = {The theory of Havens},
origtitle = {\foreignlanguage{polutonikogreek}{Θεορια τον μετεωρον}},
}
\end{filecontents}


It could be a nice idea to try and switch the language of the origtitle field based on the origlanguage information, but origlanguage is a list now, so that becomes tricky.
Alternatively, you could look into titleaddon, which is used in all standard styles and therefore available without additional modification.