Add `JSON_ASCII` option to remove unneeded decoding on client side

docs/config.rst

@@ -142,6 +142,11 @@ The following configuration values are used internally by Flask:
 ``PREFERRED_URL_SCHEME``          The URL scheme that should be used for
                                   URL generation if no URL scheme is
                                   available.  This defaults to ``http``.
+``JSON_ASCII``                    By default Flask serialize object to
+                                  ascii-encoded JSON. If this is set to
+                                  ``False`` Flask will not encode to ascii
+                                  and output strings as-is, in system
+                                  encoding (i.e. in UTF-8).
 ================================= =========================================
 
 .. admonition:: More on ``SERVER_NAME``

flask/helpers.py

@@ -195,7 +195,9 @@ def get_current_user():
     if __debug__:
         _assert_have_json()
     return current_app.response_class(json.dumps(dict(*args, **kwargs),
-        indent=None if request.is_xhr else 2), mimetype='application/json')
+        indent=None if request.is_xhr else 2,
+        ensure_ascii=current_app.config.get('JSON_ASCII', True)),
+            mimetype='application/json; charset=utf-8')
 
 
 def make_response(*args):