Hard-code the LANG env var so that `make test` passes *everywhere*

Ref: The auto locale detection test failure ("ja_JP" vs "en_US") in
https://github.com/kaushalmodi/ox-hugo/issues/195#issuecomment-416486948

Makefile

@@ -1,4 +1,4 @@
-# Time-stamp: <2018-08-27 13:31:30 kmodi>
+# Time-stamp: <2018-08-28 10:45:59 kmodi>
 
 # Makefile to export org documents to md for Hugo from the command line
 # Run just "make" to see usage examples.
@@ -101,6 +101,8 @@ help:
 # version that ships with Emacs and then run the stuff in --eval that
 # loads the new Org version.. and thus we'll end up with mixed Org in
 # the load-path.
+# The LANG env var is hard-coded to "en_US.UTF-8" so that the Locale
+# auto-detection test always passes.
 emacs-batch:
 	@echo ""
 	@echo "$(ORG_FILE) ::"
@@ -109,6 +111,7 @@ emacs-batch:
         (toggle-debug-on-error)\
 	(setenv \"OX_HUGO_ELPA\" \"$(OX_HUGO_ELPA)\")\
 	(when (> (length \"$(TIMEZONE)\") 0) (setenv \"TZ\" \"$(TIMEZONE)\"))\
+        (setenv \"LANG\" \"en_US.UTF-8\")\
 	(setq-default make-backup-files nil)\
 	(load-file (expand-file-name \"setup-ox-hugo.el\" \"$(OX_HUGO_TEST_DIR)\"))\
 	)" $(ORG_FILE) \

test/site/content-org/all-posts.org

@@ -5443,10 +5443,20 @@ Post content.
 :EXPORT_FILE_NAME: locale-specified
 :EXPORT_HUGO_LOCALE: es_ES
 :END:
+#+begin_description
+Specify the locale using ~:EXPORT_HUGO_LOCALE: es_ES~ property.
+#+end_description
+Locale exporting is enabled using ~:EXPORT_HUGO_WITH_LOCALE: t~ in the
+post's parent subtree.
 *** Auto-detected locale in front-matter                        :auto_detect:
 :PROPERTIES:
 :EXPORT_FILE_NAME: locale-auto-detect
 :END:
+#+begin_description
+Locale is not specified, so is auto-detected.
+#+end_description
+Locale exporting is enabled using ~:EXPORT_HUGO_WITH_LOCALE: t~ in the
+post's parent subtree.
 ** Images                                                            :images:
 *** Single image                                                     :single:
 :PROPERTIES:

test/site/content/posts/locale-auto-detect.md

@@ -1,6 +1,10 @@
 +++
 title = "Auto-detected locale in front-matter"
+description = "Locale is not specified, so is auto-detected."
 tags = ["front-matter", "locale", "auto-detect"]
 draft = false
 locale = "en_US"
 +++
+
+Locale exporting is enabled using `:EXPORT_HUGO_WITH_LOCALE: t` in the
+post's parent subtree.

test/site/content/posts/locale-specified.md

@@ -1,6 +1,10 @@
 +++
 title = "Specified locale in front-matter"
+description = "Specify the locale using `:EXPORT_HUGO_LOCALE: es_ES` property."
 tags = ["front-matter", "locale"]
 draft = false
 locale = "es_ES"
 +++
+
+Locale exporting is enabled using `:EXPORT_HUGO_WITH_LOCALE: t` in the
+post's parent subtree.