Pass LANGUAGE to docker

Makefile

@@ -1,6 +1,4 @@
 # Volto development
-# Allow setting the language for backend-docker-start. Default is `en`.
-LANGUAGE      ?=en
 
 ### Defensive settings for make:
 #     https://tech.davis-hansson.com/p/make/
@@ -170,7 +168,7 @@ release-notes-copy-to-docs: ## Copy release notes into documentation
 
 .PHONY: backend-docker-start
 backend-docker-start: ## Starts a Docker-based backend for development
-	$(MAKE) -C "./packages/volto/" backend-docker-start -e $(LANGUAGE)
+	$(MAKE) -C "./packages/volto/" backend-docker-start
 
 .PHONY: backend-docker-detached-start
 backend-docker-detached-start: ## Starts a Docker-based backend in detached mode (daemon)

packages/volto/Makefile

@@ -13,6 +13,9 @@ MAKEFLAGS+=--no-builtin-rules
 # Project settings (read from repo root)
 include ../../variables.mk
 
+# Allow setting the language for backend-docker-start. Default is `en`.
+LANGUAGE ?=en
+
 # Recipe snippets for reuse
 
 CHECKOUT_BASENAME="$(shell basename $(shell realpath ./))"
@@ -100,7 +103,7 @@ release-notes-copy-to-docs: ## Copy release notes into documentation
 
 .PHONY: backend-docker-start
 backend-docker-start: ## Starts a Docker-based backend for development
-	docker run -it --rm --name=backend -p 8080:8080 -v volto-backend-data:/data -e SITE=Plone -e ADDONS='$(KGS)' $(DOCKER_IMAGE)
+	docker run -it --rm --name=backend -p 8080:8080 -v volto-backend-data:/data -e SITE=Plone -e ADDONS='$(KGS)' -e LANGUAGE='$(LANGUAGE)' $(DOCKER_IMAGE)
 
 .PHONY: frontend-docker-start
 frontend-docker-start: ## Starts a Docker-based frontend for development