doc: explain how to compile with language support

Now that the setup in configure.ac have been cleaned, moved the original
documentation from the 'INSTALL-WMAKER' file into the dedicated doc
'README.i18n' and updated it.

The information is completely removed from 'INSTALL-WMAKER' with an
invitation to see the new i18n doc in order to keep that file relatively
small and to avoid duplicating information, which is always harder to
maintain.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

doc/build/Translations.texi

@@ -79,8 +79,53 @@ This manual is for Window Maker, version @value{version}.
 @end ifnottex
 
 @menu
+* Enabling Languages support::  How to compile Window Maker with i18n support
 @end menu
 
 
+@c ------------------------------------------------------------------ Enabling Languages support ---
+
+@node Enabling Languages support
+@chapter Enabling Languages support
+
+@sc{Window Maker} has the possibility to be translated in many languages, but by default none of
+them will be installed, and the support for translation will not be compiled.
+
+To enable the translation capabilities, you have to specify which language(s) you want to be
+installed: this is done with the variable @env{LINGUAS} when running the @command{configure} script.
+This variable should contain the space-separated list of languages you want to install.
+
+You could for instance enable both French (@code{fr}) and Dutch (@code{nl}) with this:
+
+@example
+./configure LINGUAS="fr nl"
+@end example
+
+You can of course add any other option that you want to the @command{configure} command.
+From the moment you specify the variable, the @command{configure} script will check that you have
+the appropriate dependencies for this (basically the @code{gettext} function and the @code{libintl}
+library); when you run @command{make} to compile the project, it will also compile the translation
+(@code{mo} files) for the language(s) you asked (if available, of course), and during
+@command{make install} it will install them in the usual directory.
+
+
+@c --------------------------------------------------------------------- LINGUAS at system level ---
+@section Setting @env{LINGUAS} at system level
+
+As the variable @env{LINGUAS} is quite standard, you also have the possibility to set its value in
+the @file{config.site} file for @sc{Autoconf}.
+This file can be placed in one of these paths:
+
+@itemize @bullet
+@item @file{@emph{<prefix>}/share/config.site}
+@item @file{@emph{<prefix>}/etc/config.site}
+@end itemize
+
+This way, the same language list will be used for all the programs that use @sc{Autoconf} that you
+would compile.
+Please note that if you also specify a value on the command line, it will have precedence over the
+value in that file.
+
+
 @c ------------------------------------------------------------------------------------- The End ---
 @bye