diff --git a/build/normalize_locale_codes.sh b/build/normalize_locale_codes.sh
new file mode 100755
index 00000000000..d87272c1555
--- /dev/null
+++ b/build/normalize_locale_codes.sh
@@ -0,0 +1,18 @@
+#!/bin/bash
+#
+# Nextcloud server, apps and clients use "de_DE" for formal German in Transifex.
+# These docs historically used "de", causing translator confusion and a fragmented
+# translation memory (see https://github.com/nextcloud/documentation/issues/13370).
+#
+# Transifex is being reconfigured to sync German as "de_DE" like every other
+# Nextcloud project, but the published docs URL must stay "de" (it's linked from
+# many external sites). This folds "de_DE" into "de" right before Sphinx builds,
+# so the sync stays de_DE-native while the published path is unaffected.
+#
+# No-op until locale/de_DE actually exists.
+
+if [ -d ./locale/de_DE ]; then
+    mkdir -p ./locale/de
+    rsync -a ./locale/de_DE/ ./locale/de/
+    rm -rf ./locale/de_DE
+fi
diff --git a/user_manual/Makefile b/user_manual/Makefile
index 240d4c7d19d..0976b056d67 100644
--- a/user_manual/Makefile
+++ b/user_manual/Makefile
@@ -49,6 +49,7 @@ clean:
 
 html:
 	../build/change_file_extension.sh
+	../build/normalize_locale_codes.sh
 	make html-all merge-folders
 
 html-all: $(foreach lang, $(LANGS), html-allow-warnings-lang-$(lang))