--- a/toolkit/locales/l10n.mk
+++ b/toolkit/locales/l10n.mk
@@ -172,18 +172,40 @@ 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))
 
+# Clone a l10n repository, either via hg or git
+# Make this a variable as it's embedded in a sh conditional
+ifeq ($(VCS_CHECKOUT_TYPE),hg)
+L10N_CO = $(HG) --cwd $(L10NBASEDIR) clone https://hg.mozilla.org/l10n-central/$(AB_CD)/
+else
+ifeq ($(VCS_CHECKOUT_TYPE),git)
+L10N_CO = $(GIT) -C $(L10NBASEDIR) clone hg://hg.mozilla.org/l10n-central/$(AB_CD)/
+else
+L10N_CO = $(error You need to use either hg or git)
+endif
+endif
+
+
 merge-%: IS_LANGUAGE_REPACK=1
+merge-%: AB_CD=$*
 merge-%:
+# For nightly builds, we automatically check out missing localizations
+# from l10n-central.
+ifdef NIGHTLY_BUILD
+	@if  ! test -d $(L10NBASEDIR)/$(AB_CD) ; then \
+		$(NSINSTALL) -D $(L10NBASEDIR) ; \
+		$(L10N_CO) ; \
+	fi
+endif
 	$(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

--- a/toolkit/moz.configure
+++ b/toolkit/moz.configure
@@ -81,22 +81,30 @@ include('../build/moz.configure/rust.con
 
 # L10N
 # ==============================================================
 option('--with-l10n-base', nargs=1, env='L10NBASEDIR',
        help='Path to l10n repositories')
 
 @depends('--with-l10n-base')
 @imports(_from='os.path', _import='isdir')
+@imports(_from='os.path', _import='expanduser')
+@imports(_from='os', _import='environ')
 def l10n_base(value):
     if value:
         path = value[0]
         if not isdir(path):
             die("Invalid value --with-l10n-base, %s doesn't exist", path)
-        return os.path.realpath(os.path.abspath(path))
+    else:
+        path = os.path.join(
+            environ.get(
+                'MOZBUILD_STATE_PATH',
+                expanduser(os.path.join('~', '.mozbuild'))),
+            'l10n-central')
+    return os.path.realpath(os.path.abspath(path))
 
 set_config('L10NBASEDIR', l10n_base)
 
 
 # Default toolkit
 # ==============================================================
 # Normally, we'd want to use the `default` field on the option, but that
 # requires --target to be resolved at --help time, which requires to run