Missing space after "guillemets"

If you want to get the correct spacing between \fg and a word following it, you have to add an empty statement {} after it.

This because the definition of \fg in frenchb.ldf resorts to the definition of \FB@fg which is

\DeclareRobustCommand*{\FB@fg}{\ifdim\lastskip>\z@\unskip\fi
                           \FBguill@spacing\guillemotright\xspace}

As you can see, this command takes no arguments, so it inserts no space after it, if you don't break the command explicitly with {} (or \ which is not advisable).

The behavior is the same as if you define a new command with no arguments

\newcommand{\qqq}{qqq}

and use it as

\qqq word

MWE

\documentclass[11pt]{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[french]{babel}
\begin{document}
Lalala \og blabla \fg{} lalala.
\end{document} 

You can also notice that the definition of \FB@fg contains an \xspace at its end. This means that, if you don't want to insert {} each time you use \fg, you can simply load the package xspace.

The following MWE gives the same result as in the above picture:

\documentclass[11pt]{scrartcl}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[french]{babel}
\usepackage{xspace}
\begin{document}
Lalala \og blabla \fg lalala.
\end{document} 

For some reason beyond my understanding, using the keyboard guillemets « and » together with the following option in the babel package

\frenchbsetup{og=«,fg=»}  

solves the problem (and elegantly too). Of course, one needs access to utf8 characters. Here is a full example.

\documentclass{article}
\usepackage[utf8]{inputenc} 
\usepackage[T1]{fontenc}
\usepackage[french]{babel}
\frenchbsetup{og=«,fg=»}

\begin{document}

Ceci est du «texte» entre guillemets.

\end{document}  

_{(source: univ-mlv.fr)}