DOCBOOK: entity reference into entity definition

Hello,

I'm working on a document translated to many languages. While the
structure of the document is the same for each language, I would like to
have a unique "top" sgml document referencing the chapters of a single
language.

My question is How do I parametrize the call to different languages?

I tried something like that, but it doesn't work:

<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V3.1//EN" [
<!ENTITY Lang "en" >
<!ENTITY ch-a SYSTEM "&Lang;/legal_notice_chapter.sgml">
...

the different chapters being stored in en/ fr/ es/ ... directories
depending on the language.

I want to  be able to change one single entity (Lang) in order to have
my target doc in french (for ex.)

Thanks, Camille.