🐛(i18n) same frontend and backend language using shared cookies

frontend: switch to cookie-based language selection
backend: use cookie for language

src/backend/impress/settings.py

@@ -223,6 +223,7 @@ class Base(Configuration):
 
     # Languages
     LANGUAGE_CODE = values.Value("en-us")
+    LANGUAGE_COOKIE_NAME = "docs_language"  # cookie & language is set from frontend
 
     DRF_NESTED_MULTIPART_PARSER = {
         # output of parser is converted to querydict