diff --git a/mercurial/dispatch.py b/mercurial/dispatch.py
--- a/mercurial/dispatch.py
+++ b/mercurial/dispatch.py
@@ -811,10 +811,8 @@
                 "option -R has to be separated from other options (e.g. not "
                 "-qR) and --repository may only be abbreviated as --repo!"))
 
-        if options["encoding"]:
-            encoding.encoding = options["encoding"]
-        if options["encodingmode"]:
-            encoding.encodingmode = options["encodingmode"]
+        encoding.setencoding(options["encoding"], options["encodingmode"])
+
         if options["time"]:
             def get_times():
                 t = os.times()
diff --git a/mercurial/encoding.py b/mercurial/encoding.py
--- a/mercurial/encoding.py
+++ b/mercurial/encoding.py
@@ -581,6 +581,20 @@
         r += c
     return r
 
+# used by i18n to clean its message cache after encoding change
+setencodingcallbacks = []
+
+def setencoding(newencoding=None, newencodingmode=None):
+    global encoding, encodingmode
+    oldvalues = (encoding, encodingmode)
+    if newencoding:
+        encoding = newencoding
+    if newencodingmode:
+        encodingmode = newencodingmode
+    if oldvalues != (encoding, encodingmode):
+        for func in setencodingcallbacks:
+            func()
+
 if pycompat.ispy3:
     class strio(io.TextIOWrapper):
         """Wrapper around TextIOWrapper that respects hg's encoding assumptions.
diff --git a/mercurial/hgweb/hgweb_mod.py b/mercurial/hgweb/hgweb_mod.py
--- a/mercurial/hgweb/hgweb_mod.py
+++ b/mercurial/hgweb/hgweb_mod.py
@@ -320,7 +320,7 @@
         rctx = requestcontext(self, repo)
 
         # This state is global across all threads.
-        encoding.encoding = rctx.config('web', 'encoding')
+        encoding.setencoding(rctx.config('web', 'encoding'))
         rctx.repo.ui.environ = req.env
 
         if rctx.csp:
diff --git a/mercurial/hgweb/hgwebdir_mod.py b/mercurial/hgweb/hgwebdir_mod.py
--- a/mercurial/hgweb/hgwebdir_mod.py
+++ b/mercurial/hgweb/hgwebdir_mod.py
@@ -173,7 +173,7 @@
 
         self.repos = repos
         self.ui = u
-        encoding.encoding = self.ui.config('web', 'encoding')
+        encoding.setencoding(self.ui.config('web', 'encoding'))
         self.style = self.ui.config('web', 'style')
         self.templatepath = self.ui.config('web', 'templates', untrusted=False)
         self.stripecount = self.ui.config('web', 'stripes')
diff --git a/mercurial/i18n.py b/mercurial/i18n.py
--- a/mercurial/i18n.py
+++ b/mercurial/i18n.py
@@ -60,6 +60,8 @@
 
 _msgcache = {}
 
+encoding.setencodingcallbacks.append(_msgcache.clear)
+
 def gettext(message):
     """Translate message.
 
diff --git a/tests/test-i18n.t b/tests/test-i18n.t
--- a/tests/test-i18n.t
+++ b/tests/test-i18n.t
@@ -48,3 +48,23 @@
   $ $PYTHON check-translation.py *.po
   $ $PYTHON check-translation.py --doctest
   $ cd $TESTTMP
+
+Check i18n cache gets invalidated correctly with encoding change:
+
+  $ cat > $TESTTMP/encodingchange.py << EOF
+  > from mercurial import encoding, registrar
+  > from mercurial.i18n import _
+  > cmdtable = {}
+  > command = registrar.command(cmdtable)
+  > @command('encodingchange', norepo=True)
+  > def encodingchange(ui):
+  >     for encode in ('ascii', 'UTF-8', 'ascii', 'UTF-8'):
+  >         encoding.setencoding(encode)
+  >         ui.write('%s\n' % encoding.jsonescape(_('(EXPERIMENTAL)')))
+  > EOF
+
+  $ LANGUAGE=ja hg --config extensions.encodingchange=$TESTTMP/encodingchange.py encodingchange
+  (?????)
+  (\xe5\xae\x9f\xe9\xa8\x93\xe7\x9a\x84\xe5\xae\x9f\xe8\xa3\x85) (esc)
+  (?????)
+  (\xe5\xae\x9f\xe9\xa8\x93\xe7\x9a\x84\xe5\xae\x9f\xe8\xa3\x85) (esc)