doc: make i18n man and html

Allow overwrite LANGUAGE and LC_ALL make variables, for make i18n man and html.
After this patch, we can make i18n man and html by following command:

  $ make clean all LANGUAGE=ja

Makefile

@@ -10,6 +10,8 b' PYTHON=python'
 PURE=
 PYFILES:=$(shell find mercurial hgext doc -name '*.py')
 DOCFILES=mercurial/help/*.txt
+export LANGUAGE=C
+export LC_ALL=C
 
 # Set this to e.g. "mingw32" to use a non-default compiler.
 COMPILER=

doc/Makefile

@@ -9,8 +9,7 b' INSTALL=install -c -m 644'
 PYTHON=python
 RSTARGS=
 
-export LANGUAGE=C
-export LC_ALL=C
+export HGENCODING=UTF-8
 
 all: man html