[WIP] localize guix.texi

[Julien Lepiller]

Hi, this new version of the patch takes your feedback into account. I tried
to put guix.fr.texi in dist_info_TEXINFOS, but automake didn't generate the
rules for building guix.fr.info in that case.

I don't like the fact that both the po and texi files are commited and will
be changed together whenever someone makes a change in the manual.

The French translation is just the beginning: I translated only the Introduction
and the Installation pages. Also, I used the po file to add the
@documentlanguage macro to change the texinfo strings to French (like "see",
"next page", "up", etc).

There's an issue with changes that add references: when that happens, the
default translation would be the English version, so the reference would be
to the English name (for instance, @pxref{Invoking guix package}), resulting
in a build failure when the name has been changed (for instance
"Invoquer guix package" in French). I would like to be able to localize the
section name because it appears in link names. Any ideas?