Cfrgtkky

 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}



    addbibresource{myfile.bib}



    begin{document}



    Test.cite{TH}



    end{document}

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}



addbibresource{jobname.bib}



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 field have been superseded by the related features, but here it might make sense.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}%

  setunit{addspace=space}%

  printfield{origtitle}%

  newunit

  printfield{titleaddon}}



usepackage{filecontents}

begin{filecontents}{jobname.bib}

@book{th,

  title     = {The theory of Havens},

  origtitle = {foreignlanguage{polutonikogreek}{Θεορια τον μετεωρον}},

}

end{filecontents}



addbibresource{jobname.bib}



begin{document}

Lorem autocite{th}



printbibliography

end{document}

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.