Problem with babel package

Question

In the following code when I am using \usepackage[greek,english]{babel} everything works fine until I try to use \selectlanguage{greek} then I get that error and those warnings:

! Corrupted NFSS tables.wrong@fontshape ...message {Corrupted NFSS tables}error@fontshape else let f... \selectlanguage{greek}

Font shape LGR/GFSArtemisia(0)/m/n' undefined(Font) usingLGR/cmr/m/n' instead

Font shape LGR/GFSArtemisia(0)/m/n' undefined(Font) usingEU1/lmr/m/n' instead

Some font shapes were not available, defaults substituted.

On the other hand when I am trying to use \usepackage[engish,greek]{babel} I will not even bother to mention how many problems I get!

I use XeLaTeX and Texmaker.

What is going on? How can I fix this?

This is the worst case using \usepackage[engish,greek]{babel}.

\documentclass[12pt]{article}

\usepackage[top=0.7in, bottom=1.2in, left=0.8in, right=0.8in]{geometry}

\setlength{\parindent}{0cm}

\usepackage[english,greek]{babel}

\usepackage{fontspec}

\setmainfont[
  Ligatures=TeX,
  Extension=.otf,
  UprightFont=*,
  BoldFont=*Bold,
  ItalicFont=*It,
  BoldItalicFont=*BoldIt,
  Mapping=tex-text,
]{GFSArtemisia}

\setsansfont[Mapping=tex-text]{GFSArtemisia.otf}

\usepackage[fleqn]{amsmath}

\usepackage{unicode-math}

\newcommand{\en}[1]{\foreignlanguage{english}{#1}}

\begin{document}

Ένα πρότυπο (ή αντικείμενο) καθορίζεται από ένα διάνυσμα μετρήσεων που ονομάζονται
χαρακτηριστικά. Το διάνυσμα ονομάζεται διάνυσμα χαρακτηριστικών.

$\overline{x}=\left[
\begin{array}{c}
\text{Feature1}\\
\text{Feature2}\\
\text{Feature2}\\
\end{array}
\right]$

\end{document}

Answer 1

The error is in loading babel before fontspec, which sets the document encoding to EU1 (with XeLaTeX) or EU2 (with LuaLaTeX). When this fact is known, babel is able to load the Greek language adapting it for those engines, without doing the tricks it has to do with the LGR encoding for pdflatex.

This might be improved in babel, but the remedy is very simple:

load fontspec before babel

Here's a polished up version:

\documentclass[12pt]{article}

\usepackage[fleqn]{amsmath}
\usepackage{unicode-math}
\usepackage[english,greek]{babel}

\setmainfont[
  Ligatures=TeX,
  Extension=.otf,
  UprightFont=*,
  BoldFont=*Bold,
  ItalicFont=*It,
  BoldItalicFont=*BoldIt,
  Mapping=tex-text,
]{GFSArtemisia}

\begin{document}

Ένα πρότυπο (ή αντικείμενο) καθορίζεται από ένα διάνυσμα μετρήσεων που ονομάζονται
χαρακτηριστικά. Το διάνυσμα ονομάζεται διάνυσμα χαρακτηριστικών.

$\overline{x}=\left[
\begin{array}{c}
\text{Feature1}\\
\text{Feature2}\\
\text{Feature2}\\
\end{array}
\right]$

\end{document}

Note that unicode-math also loads fontspec; however, using Latin Modern Math with Artemisia is not really the best, in my opinion.

Also declaring Artemisia as the sans serif font is questionable.