Fix i18n: message catalogs were reset on each initialization

CHANGES

@@ -47,6 +47,8 @@ Features added
 Bugs fixed
 ----------
 
+* i18n: message catalogs were reset on each initialization
+
 Testing
 --------
 

sphinx/locale/__init__.py

@@ -214,7 +214,7 @@ def init(locale_dirs, language, catalog='sphinx', namespace='general'):
     global translators
     translator = translators.get((namespace, catalog))
     # ignore previously failed attempts to find message catalogs
-    if isinstance(translator, NullTranslations):
+    if translator.__class__ is NullTranslations:
         translator = None
     # the None entry is the system's default locale path
     has_translation = True

tests/test_locale.py

@@ -41,9 +41,8 @@ def test_init(rootdir):
     assert _('Hello sphinx') == 'Hello sphinx'  # nothing changed here
     assert _('Hello reST') == 'Hello reST'
 
-    # load both locale1 and locale2
-    locale.init([rootdir / 'test-locale' / 'locale1',
-                 rootdir / 'test-locale' / 'locale2'], 'en', 'myext')
+    # load locale2 in addition
+    locale.init([rootdir / 'test-locale' / 'locale2'], 'en', 'myext')
     _ = locale.get_translation('myext')
     assert _('Hello world') == 'HELLO WORLD'
     assert _('Hello sphinx') == 'HELLO SPHINX'