--- a/build-aux/texinfo.tex
+++ b/build-aux/texinfo.tex
@@ -3,7 +3,7 @@
 % Load plain if necessary, i.e., if running under initex.
 \expandafter\ifx\csname fmtname\endcsname\relax\input plain\fi
 %
-\def\texinfoversion{2007-06-16.10}
+\def\texinfoversion{2007-06-20.13}
 %
 % Copyright (C) 1985, 1986, 1988, 1990, 1991, 1992, 1993, 1994, 1995,
 % 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006,
@@ -7434,22 +7434,41 @@
 
 % @documentlanguage is usually given very early, just after
 % @setfilename.  If done too late, it may not override everything
-% properly.  Single argument is the language abbreviation.
-% It would be nice if we could set up a hyphenation file here.
-%
-\parseargdef\documentlanguage{%
+% properly.  Single argument is the language (de) or locale (de_DE)
+% abbreviation.  It would be nice if we could set up a hyphenation file.
+%
+{
+  \catcode`\_ = \active
+  \globaldefs=1
+\parseargdef\documentlanguage{\begingroup
+  \let_=\normalunderscore  % normal _ character for filenames
   \tex % read txi-??.tex file in plain TeX.
-    % Read the file if it exists.
+    % Read the file by the name they passed if it exists.
     \openin 1 txi-#1.tex
     \ifeof 1
-      \errhelp = \nolanghelp
-      \errmessage{Cannot read language file txi-#1.tex}%
+      \documentlanguagetrywithoutunderscore{#1_\finish}%
     \else
       \input txi-#1.tex
     \fi
     \closein 1
   \endgroup
-}
+\endgroup}
+}
+%
+% If they passed de_DE, and txi-de_DE.tex doesn't exist,
+% try txi-de.tex.
+% 
+\def\documentlanguagetrywithoutunderscore#1_#2\finish{%
+  \openin 1 txi-#1.tex
+  \ifeof 1
+    \errhelp = \nolanghelp
+    \errmessage{Cannot read language file txi-#1.tex}%
+  \else
+    \input txi-#1.tex
+  \fi
+  \closein 1
+}
+%
 \newhelp\nolanghelp{The given language definition file cannot be found or
 is empty.  Maybe you need to install it?  In the current directory
 should work if nowhere else does.}