--- a/cubicweb/req.py	Wed Nov 02 15:59:39 2016 +0100
+++ b/cubicweb/req.py	Thu Nov 03 15:00:01 2016 +0100
@@ -76,6 +76,7 @@
         self.user = None
         self.local_perm_cache = {}
         self._ = text_type
+        self.lang = None
 
     def _set_user(self, orig_user):
         """set the user for this req_session_base
@@ -293,9 +294,13 @@
             path = kwargs.pop('_restpath')
         else:
             path = method
+        language = ''
+        if self.lang and self.vreg.config.get('language-mode') == 'url-prefix':
+            language = '%s/' % self.lang
         if not kwargs:
-            return u'%s%s' % (base_url, path)
-        return u'%s%s?%s' % (base_url, path, self.build_url_params(**kwargs))
+            return u'%s%s%s' % (base_url, language, path)
+        return u'%s%s%s?%s' % (base_url, language, path,
+                               self.build_url_params(**kwargs))
 
     def build_url_params(self, **kwargs):
         """return encoded params to incorporate them in a URL"""

--- a/cubicweb/test/unittest_req.py	Wed Nov 02 15:59:39 2016 +0100
+++ b/cubicweb/test/unittest_req.py	Thu Nov 03 15:00:01 2016 +0100
@@ -19,6 +19,7 @@
 from logilab.common.testlib import TestCase, unittest_main
 from cubicweb import ObjectNotFound
 from cubicweb.req import RequestSessionBase, FindEntityError
+from cubicweb.devtools import ApptestConfiguration
 from cubicweb.devtools.testlib import CubicWebTC
 from cubicweb import Unauthorized
 
@@ -44,6 +45,31 @@
         self.assertRaises(AssertionError, req.build_url, 'one', 'two not allowed')
         self.assertRaises(AssertionError, req.build_url, 'view', test=None)
 
+    def test_build_url_language_from_url(self):
+        # need req.vreg.config to exist because lang is read in it at set_language() call
+        vreg = MockVReg()
+        vreg.config.global_set_option('language-mode', 'url-prefix')
+        req = RequestSessionBase(vreg)
+        req.base_url = lambda secure=None: 'http://testing.fr/cubicweb/'
+        self.assertIsNone(req.lang)  # language unset yet.
+        self.assertEqual(req.build_url(), 'http://testing.fr/cubicweb/view')
+        self.assertEqual(req.build_url('foo'), 'http://testing.fr/cubicweb/foo')
+        req.set_language('fr')
+        self.assertEqual(req.lang, 'fr')
+        self.assertEqual(req.build_url(), 'http://testing.fr/cubicweb/fr/view')
+        self.assertEqual(req.build_url('foo'), 'http://testing.fr/cubicweb/fr/foo')
+        req.set_language('en')
+        self.assertEqual(req.lang, 'en')
+        self.assertEqual(req.build_url(), 'http://testing.fr/cubicweb/en/view')
+        self.assertEqual(req.build_url('foo'), 'http://testing.fr/cubicweb/en/foo')
+        # no language prefix in URL
+        vreg.config.global_set_option('language-mode', '')
+        self.assertEqual(req.build_url(), 'http://testing.fr/cubicweb/view')
+        self.assertEqual(req.build_url('foo'), 'http://testing.fr/cubicweb/foo')
+        req.set_language('fr')
+        self.assertEqual(req.build_url(), 'http://testing.fr/cubicweb/view')
+        self.assertEqual(req.build_url('foo'), 'http://testing.fr/cubicweb/foo')
+
     def test_ensure_no_rql(self):
         req = RequestSessionBase(None)
         self.assertEqual(req.ensure_ro_rql('Any X WHERE X is CWUser'), None)
@@ -52,6 +78,13 @@
         self.assertRaises(Unauthorized, req.ensure_ro_rql, '   SET X login "toto" WHERE X is CWUser   ')
 
 
+class MockVReg(object):
+    """Fake VReg with just a basic config in it.
+    """
+    def __init__(self):
+        self.config = ApptestConfiguration('data', __file__)
+
+
 class RequestCWTC(CubicWebTC):
 
     def test_base_url(self):
@@ -149,5 +182,6 @@
             with self.assertRaises(NotImplementedError):
                 req.find('CWUser', in_group=[1, 2])
 
+
 if __name__ == '__main__':
     unittest_main()

--- a/cubicweb/web/request.py	Wed Nov 02 15:59:39 2016 +0100
+++ b/cubicweb/web/request.py	Thu Nov 03 15:00:01 2016 +0100
@@ -915,25 +915,25 @@
     def html_content_type(self):
         return 'text/html'
 
+    def negotiated_language(self):
+        self.headers_out.addHeader('Vary', 'Accept-Language')
+        for lang in self.header_accept_language():
+            if lang in self.translations:
+                return lang
+        return None
+
     def set_user_language(self, user):
         vreg = self.vreg
         if user is not None:
             try:
-                # 1. user-specified language
                 lang = vreg.typed_value('ui.language', user.properties['ui.language'])
                 self.set_language(lang)
                 return
             except KeyError:
                 pass
-        if vreg.config.get('language-negociation', False):
-            # 2. http accept-language
-            self.headers_out.addHeader('Vary', 'Accept-Language')
-            for lang in self.header_accept_language():
-                if lang in self.translations:
-                    self.set_language(lang)
-                    return
-        # 3. site's default language
-        self.set_default_language(vreg)
+        # site's default language
+        if self.lang is None:
+            self.set_default_language(vreg)
 
 
 def _cnx_func(name):

--- a/cubicweb/web/test/unittest_application.py	Wed Nov 02 15:59:39 2016 +0100
+++ b/cubicweb/web/test/unittest_application.py	Thu Nov 03 15:00:01 2016 +0100
@@ -216,6 +216,87 @@
                              {'login-subject': 'required field'})
             self.assertEqual(forminfo['values'], req.form)
 
+    def test_handle_request_with_lang_fromurl(self):
+        """No language negociation, get language from URL."""
+        self.config.global_set_option('language-mode', 'url-prefix')
+        req, origsession = self.init_authentication('http')
+        self.assertEqual(req.url(), 'http://testing.fr/cubicweb/login')
+        self.assertEqual(req.lang, 'en')
+        self.app.handle_request(req)
+        newreq = self.requestcls(req.vreg, url='fr/toto')
+        self.assertEqual(newreq.lang, 'en')
+        self.assertEqual(newreq.url(), 'http://testing.fr/cubicweb/fr/toto')
+        self.app.handle_request(newreq)
+        self.assertEqual(newreq.lang, 'fr')
+        self.assertEqual(newreq.url(), 'http://testing.fr/cubicweb/fr/toto')
+        # unknown language
+        newreq = self.requestcls(req.vreg, url='unknown-lang/cwuser')
+        result = self.app.handle_request(newreq)
+        self.assertEqual(newreq.lang, 'en')
+        self.assertEqual(newreq.url(), 'http://testing.fr/cubicweb/unknown-lang/cwuser')
+        self.assertIn('this resource does not exist',
+                      result.decode('ascii', errors='ignore'))
+        # no prefix
+        newreq = self.requestcls(req.vreg, url='cwuser')
+        result = self.app.handle_request(newreq)
+        self.assertEqual(newreq.lang, 'en')
+        self.assertEqual(newreq.url(), 'http://testing.fr/cubicweb/cwuser')
+        self.assertNotIn('this resource does not exist',
+                         result.decode('ascii', errors='ignore'))
+
+    def test_handle_request_with_lang_negotiated(self):
+        """Language negociated, normal case."""
+        self.config.global_set_option('language-mode', 'http-negotiation')
+        orig_translations = self.config.translations.copy()
+        self.config.translations = {'fr': (text_type, text_type),
+                                    'en': (text_type, text_type)}
+        try:
+            headers = {'Accept-Language': 'fr'}
+            with self.admin_access.web_request(headers=headers) as req:
+                self.app.handle_request(req)
+            self.assertEqual(req.lang, 'fr')
+        finally:
+            self.config.translations = orig_translations
+
+    def test_handle_request_with_lang_negotiated_prefix_in_url(self):
+        """Language negociated, unexpected language prefix in URL."""
+        self.config.global_set_option('language-mode', 'http-negotiation')
+        with self.admin_access.web_request(url='fr/toto') as req:
+            result = self.app.handle_request(req)
+        self.assertIn('this resource does not exist',  # NotFound.
+                      result.decode('ascii', errors='ignore'))
+
+    def test_handle_request_no_lang_negotiation_fixed_language(self):
+        """No language negociation, "ui.language" fixed."""
+        self.config.global_set_option('language-mode', '')
+        vreg = self.app.vreg
+        self.assertEqual(vreg.property_value('ui.language'), 'en')
+        props = []
+        try:
+            with self.admin_access.cnx() as cnx:
+                props.append(cnx.create_entity('CWProperty', value=u'de',
+                                               pkey=u'ui.language').eid)
+                cnx.commit()
+            self.assertEqual(vreg.property_value('ui.language'), 'de')
+            headers = {'Accept-Language': 'fr'}  # should not have any effect.
+            with self.admin_access.web_request(headers=headers) as req:
+                self.app.handle_request(req)
+            # user has no "ui.language" property, getting site's default.
+            self.assertEqual(req.lang, 'de')
+            with self.admin_access.cnx() as cnx:
+                props.append(cnx.create_entity('CWProperty', value=u'es',
+                                               pkey=u'ui.language',
+                                               for_user=cnx.user).eid)
+                cnx.commit()
+            with self.admin_access.web_request(headers=headers) as req:
+                self.app.handle_request(req)
+            self.assertEqual(req.lang, 'es')
+        finally:
+            with self.admin_access.cnx() as cnx:
+                for peid in props:
+                    cnx.entity_from_eid(peid).cw_delete()
+                cnx.commit()
+
     def test_validation_error_dont_loose_subentity_data_repo(self):
         """test creation of two linked entities
 

--- a/cubicweb/web/test/unittest_request.py	Wed Nov 02 15:59:39 2016 +0100
+++ b/cubicweb/web/test/unittest_request.py	Thu Nov 03 15:00:01 2016 +0100
@@ -83,6 +83,15 @@
         self.assertEqual('http://babar.com/', req.base_url(False))
         self.assertEqual('https://toto.com/', req.base_url(True))
 
+    def test_negotiated_language(self):
+        vreg = type('DummyVreg', (object,), {})()
+        vreg.config = FakeConfig()
+        vreg.config.translations = {'fr': (None, None), 'en': (None, None)}
+        headers = {
+            'Accept-Language': 'fr,fr-fr;q=0.8,en-us;q=0.5,en;q=0.3',
+        }
+        req = CubicWebRequestBase(vreg, https=False, headers=headers)
+        self.assertEqual(req.negotiated_language(), 'fr')
 
 
 if __name__ == '__main__':

--- a/cubicweb/web/views/urlpublishing.py	Wed Nov 02 15:59:39 2016 +0100
+++ b/cubicweb/web/views/urlpublishing.py	Thu Nov 03 15:00:01 2016 +0100
@@ -115,7 +115,21 @@
         :raise NotFound: if no handler is able to decode the given path
         """
         parts = [part for part in path.split('/')
-                 if part != ''] or (self.default_method,)
+                 if part != ''] or [self.default_method]
+        language_mode = self.vreg.config.get('language-mode')
+        if (language_mode == 'url-prefix'
+                and parts and parts[0] in self.vreg.config.available_languages()):
+            # language from URL
+            req.set_language(parts.pop(0))
+            path = '/'.join(parts)
+            # if parts only contains lang, use 'view' default path
+            if not parts:
+                parts = (self.default_method,)
+        elif language_mode in ('http-negotiation', 'url-prefix'):
+            # negotiated language
+            lang = req.negotiated_language()
+            if lang:
+                req.set_language(lang)
         if req.form.get('rql'):
             if parts[0] in self.vreg['controllers']:
                 return parts[0], None

--- a/cubicweb/web/webconfig.py	Wed Nov 02 15:59:39 2016 +0100
+++ b/cubicweb/web/webconfig.py	Thu Nov 03 15:00:01 2016 +0100
@@ -178,11 +178,13 @@
           'group': 'web', 'level': 2,
           }),
 
-        ('language-negociation',
-         {'type' : 'yn',
-          'default': True,
-          'help': 'use Accept-Language http header to try to set user '\
-          'interface\'s language according to browser defined preferences',
+        ('language-mode',
+         {'type' : 'choice',
+          'choices': ('http-negotiation', 'url-prefix', ''),
+          'default': 'http-negotiation',
+          'help': ('source for interface\'s language detection. '
+                   'If set to "http-negotiation" the Accept-Language HTTP header will be used,'
+                   ' if set to "url-prefix", the URL will be inspected for a short language prefix.'),
           'group': 'web', 'level': 2,
           }),