Encoding error when changing from LaTeX to XeTeX engine in Emacs AucTeX

Question

The following MWE works fine when compiling with the LaTeX engine (AUCTeX 11.86)

\documentclass{article}

\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}

\begin{document}
áéíóúñ
\end{document}

However, with the XeTeX engine I get the following error:

ERROR: Package inputenc Error: Unicode char \u8:áéí not set up for use with LaTeX

--- TeX said ---
.

See the inputenc package documentation for explanation.

If I change the input encoding to Latin-1 the document compiles correctly. Why is XeTeX rejecting the UTF-8 encoding?

I tried the suggestions given in Typeset directly in Spanish and in AUCTeX and Xetex but neither help.

Answer 1

\usepackage[utf8]{inputenc} with XeLaTeX is wrong, because XeTeX directly reads UTF-8 and that package confuses the way it interprets the characters.

Files input to Xe(La)TeX should always be UTF-8 encoded (UTF-16 and UTF-32 work too, but UTF-8 seems preferable). There is the possibility to specify a different input encoding, but this is for legacy documents and is not what you want.

If you want a document that typesets both with pdflatex and xelatex, then you can try

\documentclass{article}

\usepackage{ifxetex}

\ifxetex
  \usepackage{fontspec}
\else
  \usepackage[T1]{fontenc}
  \usepackage[utf8]{inputenc}
  \usepackage{lmodern}
\fi

\begin{document}
áéíóúñ
\end{document}

and save the file as UTF-8. You can use babel in both cases, since Spanish is based on the Latin alphabet; however, switching to Polyglossia for XeLaTeX may be better:

\documentclass{article}

\usepackage{ifxetex}

\ifxetex
  \usepackage{fontspec}
  \usepackage{polyglossia}
  \setmainlanguage{spanish}
\else
  \usepackage[T1]{fontenc}
  \usepackage[utf8]{inputenc}
  \usepackage[spanish]{babel}
  \usepackage{lmodern}
\fi

\begin{document}
áéíóúñ
\end{document}

Note that fontspec uses by default the OpenType version of the Latin Modern fonts.