Language name in FO

I have just started using xsl to produce fo for FOP, using FOP 0.20.5, which no longer comes with a hyphen pattern for english
(en) but does have  en_GB hyphenation. How can I make the xsl stylesheet put the  language name 'en_GB' in the
the FO. I've tried setting  "l10n.gentext.language"  to "'en_GB'" in the  customisation, but the _GB is not output
in the FO. How can I set the output language ? 
     Ian Moor
Ian W Moor                  Department of Computing,
           Imperial College.
                                     London SW7 2AZ UK.