Problem with babel and fontspec

Question

I've just had a problem on using babel with lualatex (or xelatex as well) and fontspec in the context of latin and cyrillic alphabets. Here is a minimal example:

\documentclass[11pt]{article}

\usepackage{fontspec}
\setmainfont{Heuristica}%
\usepackage[english,russian]{babel}
\newcommand*\English{\selectlanguage{english}}

\begin{document}

\textup{\Russian Мороз и солнце, день чудесный}

\English\textit{The sea is calm to-night.}
\end{document} 

And here's the resulting .pdf:

As one can see, the font used for the English part of the text is Computer Modern, not Heuristica, and there's not even italic where it's supposed to be italic.

However, if I change the order of the languages ([russian, english], everything is fine:

If I compile with pdflatex, modifying accordingly the preamble and using the type 1 versions of the fonts instead of the opentype, everything is fine whatever the order of the languages. I tried with some other fonts, same problem. Is this a bug or do I miss something?

Answer 1

egreg's answer now explains why this works which I didn't understand when I posted it as a comment.

If you load fontspec after babel, then it all works:

\documentclass[11pt]{article}

\usepackage[british,russian]{babel}
\newcommand*\English{\selectlanguage{british}}
\usepackage{fontspec}
\setmainfont{Heuristica}%
\usepackage{poetrytex}

\begin{document}

\renewcommand{\poetryheadings}{\pagestyle{myheadings} \markboth{}{}}
\English
\begin{poem}{Title}{Author\\2014}
\textup{\Russian Мороз и солнце, день чудесный}\\
\textit{The sea is calm to-night.}
\end{poem}

\end{document}

But note that the fontspec manual describes babel as 'not really supported' and recommends polyglossia (page 4). I assume they mean for languages supported by the latter which is not all of them but does include Russian.

Answer 2

The relevant part from babel.def is

\AtEndOfPackage{\edef\latinencoding{\cf@encoding}}
\AtBeginDocument{%
  \gdef\latinencoding{OT1}%
  \ifx\cf@encoding\bbl@t@one
    \xdef\latinencoding{\bbl@t@one}%
  \else
    \@ifl@aded{def}{t1enc}{\xdef\latinencoding{\bbl@t@one}}{}%
  \fi
  }

Let's see what happens: at the end of the loading of babel, the instruction

\edef\latinencoding{\cf@encoding}

is performed; since fontspec recognizes being called by XeLaTeX, it sets \cf@encoding to EU1. This might be needed from packages loaded later on.

Then, at begin document, \latinencoding is set temporarily to OT1 and then the value of \cf@encoding is examined again, because it might have changed. If it is T1, then \latinencoding is changed to T1; the same is done if t1enc.def has been loaded. Since in your case \cf@encoding is EU1 and t1enc has not been loaded, nothing happens and \latinencoding keeps its value OT1.

Indeed, you get warnings from LaTeX such as

LaTeX Font Warning: Font shape `OT1/Heuristica(0)/m/n' undefined
(Font)              using `OT1/cmr/m/n' instead on input line 11.

that is issued when \English is done, because babel executes \edef\encodingdefault{\latinencoding}, in order to be able to use Latin characters.

This doesn't happen if, more correctly, fontspec is loaded after babel, because fontspec (actually fontspec-xetex.sty) does

\tl_set_eq:NN \cyrillicencoding \g_fontspec_encoding_tl
\tl_set_eq:NN \latinencoding    \g_fontspec_encoding_tl
\AtBeginDocument
 {
  \tl_set_eq:NN \cyrillicencoding \g_fontspec_encoding_tl
  \tl_set_eq:NN \latinencoding    \g_fontspec_encoding_tl
 }

and \g_fontspec_encoding_tl is EU1, so the faulty redefinition of \latinencoding to OT1 is overridden.