[web] only set "Vary: Accept-Language" when we translate something

Some requests never go through gettext, so their result doesn't depend
on the selected language.

cubicweb/web/request.py

@@ -27,6 +27,7 @@ import http.client
 from io import BytesIO
 from urllib.parse import urlsplit, quote as urlquote
 from http.cookies import SimpleCookie
+from functools import wraps
 
 from rql.utils import rqlvar_maker
 
@@ -842,6 +843,22 @@ class _CubicWebRequestBase(RequestSessionBase):
                 return lang
         return None
 
+    def set_language_from_req(self, lang):
+        # The chosen language comes from the request's Accept-Language header:
+        # if we ever translate something, set Vary to indicate that our response
+        # depends on that request header.
+        self.set_language(lang)
+
+        def add_headers_on_call(func):
+            @wraps(func)
+            def tr(*args, **kwargs):
+                self.headers_out.addHeader('Vary', 'Accept-Language')
+                return func(*args, **kwargs)
+            return tr
+        self._ = add_headers_on_call(self._)
+        self.__ = add_headers_on_call(self.__)
+        self.pgettext = add_headers_on_call(self.pgettext)
+
 
 def _cnx_func(name):
     def proxy(req, *args, **kwargs):

cubicweb/web/views/urlpublishing.py

@@ -129,7 +129,10 @@ class URLPublisherComponent(component.Component):
             # negotiated language
             lang = req.negotiated_language()
             if lang:
-                req.set_language(lang)
+                if language_mode == 'http-negotiation':
+                    req.set_language_from_req(lang)
+                else:
+                    req.set_language(lang)
         origin_rql = req.form.get('rql')
         if origin_rql:
             if parts[0] in self.vreg['controllers']: