Problem with Polyglossia setting \textgreek after update to TeX Live 2018

Question

I used to set an specific font for greek text using polyglossia with \newfontfamily{\textgreek}[Ligatures=TeX]{Vusillus Old Face}.

After I updated to TeX Live 2018 all my documents wouldn't compile anymore throwing the error:

LaTeX error: "xparse/command-already-defined" Command '\textgreek' already defined! For immediate help type H .

This is truly unfortunate, because that means the update breaks compatibility to a couple of hundred of my documents. However, my questions are:

1. What happened in polyglossia, that I cant use the above mentioned
   command anymore, to set the font for a specific language?
2. How am I supposed to do this now?
3. How could I make my old documents compile again the easiest way?

By the way, I am using polyglossia due to its compatibility with XeLaTeX. Here is a minimal not working example:

\documentclass[a4paper,10pt,twoside]{memoir}
\usepackage{polyglossia}
\setmainlanguage[spelling=old,babelshorthands=true,script=latin]{german}
\setotherlanguage[variant=polytonic]{greek}
\newfontfamily{\textgreek}[Ligatures=TeX]{Vusillus Old Face}

\begin{document}
textgreek{πάντα ῥεῖ} – Alles fließt.
\end{document}

Answer 1

fontspec recently changed the behaviour of \newfontfamily. \newfontfamily<cmd> now throws an error if <cmd> is defined. Earlier \newfontfamily would behave like \setfontfamily behaves now. It would simply overwrite the old definition of the command. (See p. 9 of the fontspec documentation).

As David Carlisle explains in the comments \textgreek is a command defined by polyglossia to typeset Greek. (polyglossia defines \text<language> for all loaded languages. See also p. 5 of the polyglossia documentation.)

So when you get to

\newfontfamily{\textgreek}[Ligatures=TeX]{Vusillus Old Face}

new versions of fontspec throw an error, because \textgreek is already defined. Older version of fontspec will have overwritten polyglossia's definition of \textgreek, which probably meant that you did not actually get proper language switching.

The correct way to specify the font for Greek is by redefining \greekfont. See also pp. 6-7 of the polyglossia documentation.

\documentclass[a4paper,10pt,twoside]{memoir}
\usepackage{polyglossia}
\setmainlanguage[spelling=old,babelshorthands=true,script=latin]{german}
\setotherlanguage[variant=polytonic]{greek}
\newfontfamily{\greekfont}[Ligatures=TeX]{Linux Libertine O}

\begin{document}
\textgreek{πάντα ῥεῖ} – Alles fließt.
\end{document}

You could also use babel instead of polyglossia. babel supports LuaLaTeX and XeLaTeX and is actively being developed. (polyglossia development seems to have stalled recently.) For western languages with Latin scripts polyglossia is barely better than babel. For Greek babel also gives very good results. Even for RTL languages recent babel development looks very promising.

\documentclass[a4paper,10pt,twoside]{memoir}
\usepackage{fontspec}
\usepackage[polutonikogreek,german]{babel}
\babelfont{rm}{Latin Modern Roman}
\babelfont[polutonikogreek]{rm}{Linux Libertine O}

\begin{document}
\foreignlanguage{polutonikogreek}{πάντα ῥεῖ} – Alles fließt.
\end{document}

The output of both examples is the same