--- a/browser/installer/windows/Makefile.in
+++ b/browser/installer/windows/Makefile.in
@@ -28,19 +28,19 @@ BRANDING_FILES = \
 	firefox64.ico \
 	wizHeader.bmp \
 	wizHeaderRTL.bmp \
 	wizWatermark.bmp \
 	$(NULL)
 
 include $(topsrcdir)/config/config.mk
 
-ifdef LOCALE_MERGEDIR
+ifdef IS_LANGUAGE_REPACK
 PPL_LOCALE_ARGS = \
-  --l10n-dir=$(LOCALE_MERGEDIR)/browser/installer \
+  --l10n-dir=$(REAL_LOCALE_MERGEDIR)/browser/installer \
   --l10n-dir=$(call EXPAND_LOCALE_SRCDIR,browser/locales)/installer \
   --l10n-dir=$(topsrcdir)/browser/locales/en-US/installer \
   $(NULL)
 else
 PPL_LOCALE_ARGS=$(call EXPAND_LOCALE_SRCDIR,browser/locales)/installer
 endif
 
 OVERRIDE_DEFAULT_GOAL := installer

--- a/browser/locales/Makefile.in
+++ b/browser/locales/Makefile.in
@@ -82,16 +82,17 @@ searchplugins:: $(list-json)
 
 $(DIST)/branding:
 	$(NSINSTALL) -D $@
 
 DEFINES += -DBOOKMARKS_INCLUDE_DIR=$(dir $(call MERGE_FILE,profile/bookmarks.inc))
 
 libs-%: AB_CD=$*
 libs-%:
+	$(if $(filter en-US,$(AB_CD)),, @$(MAKE) merge-$*)
 	$(NSINSTALL) -D $(DIST)/install
 	@$(MAKE) -C ../../toolkit/locales libs-$* XPI_ROOT_APPID='$(XPI_ROOT_APPID)'
 	@$(MAKE) -C ../../services/sync/locales AB_CD=$* XPI_NAME=locale-$*
 	@$(MAKE) -C ../../extensions/spellcheck/locales AB_CD=$* XPI_NAME=locale-$*
 ifndef RELEASE_OR_BETA
 	@$(MAKE) -C ../extensions/formautofill/locales AB_CD=$* XPI_NAME=locale-$*
 endif
 	@$(MAKE) -C ../extensions/onboarding/locales AB_CD=$* XPI_NAME=locale-$*
@@ -104,17 +105,19 @@ ifneq '$(or $(MOZ_DEV_EDITION),$(NIGHTLY
 endif
 	@$(MAKE) -C ../../intl/locales AB_CD=$* XPI_NAME=locale-$*
 	@$(MAKE) -C ../../devtools/client/locales AB_CD=$* XPI_NAME=locale-$* XPI_ROOT_APPID='$(XPI_ROOT_APPID)'
 	@$(MAKE) -B searchplugins AB_CD=$* XPI_NAME=locale-$*
 	@$(MAKE) libs AB_CD=$* XPI_NAME=locale-$* PREF_DIR=$(PREF_DIR)
 	@$(MAKE) -C $(DEPTH)/$(MOZ_BRANDING_DIRECTORY)/locales AB_CD=$* XPI_NAME=locale-$*
 
 chrome-%: AB_CD=$*
+chrome-%: IS_LANGUAGE_REPACK=1
 chrome-%:
+	$(if $(filter en-US,$(AB_CD)),, @$(MAKE) merge-$*)
 	@$(MAKE) -C ../../toolkit/locales chrome-$*
 	@$(MAKE) -C ../../services/sync/locales chrome AB_CD=$*
 	@$(MAKE) -C ../../extensions/spellcheck/locales chrome AB_CD=$*
 ifndef RELEASE_OR_BETA
 	@$(MAKE) -C ../extensions/formautofill/locales chrome AB_CD=$*
 endif
 	@$(MAKE) -C ../extensions/pocket/locale chrome AB_CD=$*
 ifndef RELEASE_OR_BETA
@@ -152,16 +155,17 @@ endif
 
 
 langpack: langpack-$(AB_CD)
 
 # This is a generic target that will make a langpack, repack ZIP (+tarball)
 # builds, and repack an installer if applicable. It is called from the
 # tinderbox scripts. Alter it with caution.
 
+installers-%: IS_LANGUAGE_REPACK=1
 installers-%: clobber-% langpack-% repackage-win32-installer-% repackage-zip-%
 	@echo 'repackaging done'
 
 ifdef MOZ_UPDATER
 # Note that we want updater.ini to be in the top directory, not the browser/
 # subdirectory, because that's where the updater is installed and runs.
 libs:: $(call MERGE_FILE,updater/updater.ini) $(call mkdir_deps,$(DIST)/bin)
 ifeq ($(OS_ARCH),WINNT)
@@ -172,44 +176,38 @@ ifeq ($(OS_ARCH),WINNT)
 else
 	cat $< | \
 	  sed -e 's/^InfoText=/Info=/' -e 's/^TitleText=/Title=/' | \
 	  sed -e 's/%MOZ_APP_DISPLAYNAME%/$(MOZ_APP_DISPLAYNAME)/' > \
 	  $(FINAL_TARGET)/../updater.ini
 endif
 endif
 
+ifndef IS_LANGPACK
 ifdef MOZ_CRASHREPORTER
 libs:: $(call MERGE_FILE,crashreporter/crashreporter-override.ini)
 	$(SYSINSTALL) $(IFLAGS1) $^ $(FINAL_TARGET)
 endif
+endif
 
 ident:
 	@printf 'fx_revision '
 	@$(PYTHON) $(topsrcdir)/config/printconfigsetting.py \
 	    $(STAGEDIST)/application.ini App SourceStamp
 	@printf 'buildid '
 	@$(PYTHON) $(topsrcdir)/config/printconfigsetting.py \
 	    $(STAGEDIST)/application.ini App BuildID
 
-merge-%: AB_CD=$*
-merge-%:
-ifdef LOCALE_MERGEDIR
-	$(RM) -rf $(LOCALE_MERGEDIR)
-	$(topsrcdir)/mach compare-locales --merge-dir $(LOCALE_MERGEDIR) $*
-endif
-	@echo
-
 # test target, depends on make package
 # try to repack x-test, with just toolkit/defines.inc being there
 l10n-check:: INNER_UNMAKE_PACKAGE=true
 l10n-check::
 	$(RM) -rf x-test
 	$(NSINSTALL) -D x-test/toolkit
 	echo '#define MOZ_LANG_TITLE Just testing' > x-test/toolkit/defines.inc
 	@# ZIP_IN='$(ZIP_IN)' will pass down the *current* value of ZIP_IN, and
 	@# we do the same for WIN32_INSTALLER_IN, which are based on MOZ_SIMPLE_PACKAGE_NAME
 	@# not being reset, overwriting the value they would get with MOZ_SIMPLE_PACKAGE_NAME
 	@# reset.
-	$(MAKE) installers-x-test L10NBASEDIR='$(PWD)' LOCALE_MERGEDIR='$(PWD)/mergedir' \
+	$(MAKE) installers-x-test L10NBASEDIR='$(PWD)' \
 	    ZIP_IN='$(ZIP_IN)' WIN32_INSTALLER_IN='$(WIN32_INSTALLER_IN)' MOZ_SIMPLE_PACKAGE_NAME=
 	$(PYTHON) $(topsrcdir)/toolkit/mozapps/installer/unpack.py $(DIST)/l10n-stage/$(MOZ_PKG_DIR)$(_RESPATH)
 	cd $(DIST)/l10n-stage && test $$(cat $(MOZ_PKG_DIR)$(_RESPATH)/update.locale) = x-test

--- a/config/config.mk
+++ b/config/config.mk
@@ -476,45 +476,43 @@ sysinstall_cmd = install_cmd
 # MOZ_UI_LOCALE directly, but use an intermediate variable that can be
 # overridden by the command line. (Besides, AB_CD is prettier).
 AB_CD = $(MOZ_UI_LOCALE)
 # Many locales directories want this definition.
 ACDEFINES += -DAB_CD=$(AB_CD)
 
 ifndef L10NBASEDIR
   L10NBASEDIR = $(error L10NBASEDIR not defined by configure)
-else
-  IS_LANGUAGE_REPACK = 1
 endif
 
 EXPAND_LOCALE_SRCDIR = $(if $(filter en-US,$(AB_CD)),$(topsrcdir)/$(1)/en-US,$(or $(realpath $(L10NBASEDIR)),$(abspath $(L10NBASEDIR)))/$(AB_CD)/$(subst /locales,,$(1)))
 
 ifdef relativesrcdir
 LOCALE_SRCDIR ?= $(call EXPAND_LOCALE_SRCDIR,$(relativesrcdir))
 endif
 
 ifdef relativesrcdir
 MAKE_JARS_FLAGS += --relativesrcdir=$(relativesrcdir)
 ifneq (en-US,$(AB_CD))
-ifdef LOCALE_MERGEDIR
-MAKE_JARS_FLAGS += --locale-mergedir=$(LOCALE_MERGEDIR)
+ifdef IS_LANGUAGE_REPACK
+MAKE_JARS_FLAGS += --locale-mergedir=$(REAL_LOCALE_MERGEDIR)
 endif
 ifdef IS_LANGUAGE_REPACK
 MAKE_JARS_FLAGS += --l10n-base=$(L10NBASEDIR)/$(AB_CD)
 endif
 else
 MAKE_JARS_FLAGS += -c $(LOCALE_SRCDIR)
 endif # en-US
 else
 MAKE_JARS_FLAGS += -c $(LOCALE_SRCDIR)
 endif # ! relativesrcdir
 
-ifdef LOCALE_MERGEDIR
+ifdef IS_LANGUAGE_REPACK
 MERGE_FILE = $(firstword \
-  $(wildcard $(LOCALE_MERGEDIR)/$(subst /locales,,$(relativesrcdir))/$(1)) \
+  $(wildcard $(REAL_LOCALE_MERGEDIR)/$(subst /locales,,$(relativesrcdir))/$(1)) \
   $(wildcard $(LOCALE_SRCDIR)/$(1)) \
   $(srcdir)/en-US/$(1) )
 else
 MERGE_FILE = $(LOCALE_SRCDIR)/$(1)
 endif
 MERGE_FILES = $(foreach f,$(1),$(call MERGE_FILE,$(f)))
 
 # These marcros are similar to MERGE_FILE, but no merging, and en-US first.

--- a/mobile/android/locales/Makefile.in
+++ b/mobile/android/locales/Makefile.in
@@ -26,36 +26,44 @@ include $(topsrcdir)/toolkit/locales/l10
 # values-*/strings.xml and raw-*/suggestedsites.json.
 # Those would be in the way of a single locale build, which this
 # target is for
 clobber-stage:
 	$(RM) -rf $(STAGEDIST)
 	$(RM) $(DEPTH)/mobile/android/base/res/values-*/strings.xml
 	$(RM) $(DEPTH)/mobile/android/base/res/raw-*/suggestedsites.json
 
+# merge if we're not en-US, using conditional function as we need
+# the current value of AB_CD
 libs-%: AB_CD=$*
 libs-%:
+	$(if $(filter en-US,$(AB_CD)),, @$(MAKE) merge-$*)
 	@$(MAKE) -C $(DEPTH)/mobile/locales libs-$*
 	@$(MAKE) libs AB_CD=$* XPI_NAME=locale-$* PREF_DIR=defaults/pref
 ifeq ($(OS_TARGET),Android)
 	@$(MAKE) -C $(DEPTH)/mobile/android/base/locales AB_CD=$* XPI_NAME=locale-$*
 endif
 
 # Tailored target to just add the chrome processing for multi-locale builds
+# merge if we're not en-US, using conditional function as we need
+# the current value of AB_CD
 chrome-%: AB_CD=$*
+chrome-%: IS_LANGUAGE_REPACK=1
 chrome-%:
+	$(if $(filter en-US,$(AB_CD)),, @$(MAKE) merge-$*)
 	@$(MAKE) -C $(DEPTH)/mobile/locales chrome-$*
 	@$(MAKE) chrome AB_CD=$*
 ifeq ($(OS_TARGET),Android)
 	@$(MAKE) -C $(DEPTH)/mobile/android/base/locales chrome-$*
 endif
 
 # This is a generic target that will make a langpack and repack tarball
 # builds. It is called from the tinderbox scripts. Alter it with caution.
 
+installers-%: IS_LANGUAGE_REPACK=1
 installers-%: clobber-stage repackage-zip-%
 	@echo 'repackaging done'
 
 # When we unpack fennec on MacOS X the platform.ini and application.ini are in slightly
 # different locations that on all other platforms
 ifeq (Darwin, $(OS_ARCH))
 GECKO_PLATFORM_INI_PATH='$(STAGEDIST)/platform.ini'
 FENNEC_APPLICATION_INI_PATH='$(STAGEDIST)/application.ini'
@@ -66,16 +74,8 @@ endif
 
 ident:
 	@printf 'gecko_revision '
 	@$(PYTHON) $(topsrcdir)/config/printconfigsetting.py $(GECKO_PLATFORM_INI_PATH) Build SourceStamp
 	@printf 'fennec_revision '
 	@$(PYTHON) $(topsrcdir)/config/printconfigsetting.py $(FENNEC_APPLICATION_INI_PATH) App SourceStamp
 	@printf 'buildid '
 	@$(PYTHON) $(topsrcdir)/config/printconfigsetting.py $(FENNEC_APPLICATION_INI_PATH) App BuildID
-
-merge-%: AB_CD=$*
-merge-%:
-ifdef LOCALE_MERGEDIR
-	$(RM) -rf $(LOCALE_MERGEDIR)
-	$(topsrcdir)/mach compare-locales --merge-dir $(LOCALE_MERGEDIR) $*
-endif
-	@echo

--- a/toolkit/locales/Makefile.in
+++ b/toolkit/locales/Makefile.in
@@ -19,16 +19,18 @@ chrome-%:
 	@$(MAKE) -C $(DEPTH)/netwerk/locales/ chrome AB_CD=$*
 	@$(MAKE) -C $(DEPTH)/dom/locales/ chrome AB_CD=$*
 	@$(MAKE) -C $(DEPTH)/security/manager/locales/ chrome AB_CD=$*
 	@$(MAKE) chrome AB_CD=$*
 
 libs:: update.locale
 	sed -e 's/%AB_CD%/$(AB_CD)/' $< > $(FINAL_TARGET)/update.locale
 
+ifndef IS_LANGPACK
 ifdef MOZ_CRASHREPORTER
 libs:: $(call MERGE_FILE,crashreporter/crashreporter.ini)
 ifeq (cocoa,$(MOZ_WIDGET_TOOLKIT))
 	$(SYSINSTALL) $(IFLAGS1) $^ $(FINAL_TARGET)/crashreporter.app/Contents/Resources
 else
 	$(SYSINSTALL) $(IFLAGS1) $^ $(FINAL_TARGET)
 endif
 endif
+endif

--- a/toolkit/locales/l10n.mk
+++ b/toolkit/locales/l10n.mk
@@ -51,16 +51,23 @@ ACDEFINES += \
 	-DMOZ_APP_ID='$(MOZ_APP_ID)' \
 	-DMOZ_APP_VERSION=$(MOZ_APP_VERSION) \
 	-DMOZ_APP_MAXVERSION=$(MOZ_APP_MAXVERSION) \
 	-DLOCALE_SRCDIR=$(abspath $(LOCALE_SRCDIR)) \
 	-DPKG_BASENAME='$(PKG_BASENAME)' \
 	-DPKG_INST_BASENAME='$(PKG_INST_BASENAME)' \
 	$(NULL)
 
+# export some global defines for l10n repacks
+BASE_MERGE:=$(CURDIR)/merge-dir
+export REAL_LOCALE_MERGEDIR=$(BASE_MERGE)/$(AB_CD)
+# is an l10n repack step:
+export IS_LANGUAGE_REPACK
+# is a language pack:
+export IS_LANGPACK
 
 clobber-%: AB_CD=$*
 clobber-%:
 	$(RM) -rf $(DIST)/xpi-stage/locale-$*
 
 
 PACKAGER_NO_LIBS = 1
 
@@ -165,19 +172,26 @@ TK_DEFINES = $(firstword \
    $(MOZILLA_DIR)/toolkit/locales/en-US/defines.inc)
 
 # Dealing with app sub dirs: If DIST_SUBDIRS is defined it contains a
 # listing of app sub-dirs we should include in langpack xpis. If not,
 # check DIST_SUBDIR, and if that isn't present, just package the default
 # chrome directory.
 PKG_ZIP_DIRS = chrome $(or $(DIST_SUBDIRS),$(DIST_SUBDIR))
 
+merge-%: IS_LANGUAGE_REPACK=1
+merge-%:
+	$(RM) -rf $(REAL_LOCALE_MERGEDIR)
+	$(MOZILLA_DIR)/mach compare-locales --l10n-base $(L10NBASEDIR) --merge-dir $(REAL_LOCALE_MERGEDIR) $*
+
 langpack-%: LANGPACK_FILE=$(ABS_DIST)/$(PKG_LANGPACK_PATH)$(PKG_LANGPACK_BASENAME).xpi
 langpack-%: AB_CD=$*
 langpack-%: XPI_NAME=locale-$*
+langpack-%: IS_LANGUAGE_REPACK=1
+langpack-%: IS_LANGPACK=1
 langpack-%: libs-%
 	@echo 'Making langpack $(LANGPACK_FILE)'
 	$(NSINSTALL) -D $(DIST)/$(PKG_LANGPACK_PATH)
 	$(call py_action,preprocessor,$(DEFINES) $(ACDEFINES) \
 	  -DTK_DEFINES=$(TK_DEFINES) -DAPP_DEFINES=$(APP_DEFINES) $(MOZILLA_DIR)/toolkit/locales/generic/install.rdf -o $(DIST)/xpi-stage/$(XPI_NAME)/install.rdf)
 	$(call py_action,zip,-C $(DIST)/xpi-stage/locale-$(AB_CD) $(LANGPACK_FILE) install.rdf $(PKG_ZIP_DIRS) chrome.manifest)
 
 # This variable is to allow the wget-en-US target to know which ftp server to download from