[fix] add missing localizable (gettext) messages to searxng.msg

To test this patch I used .. and checked the diff of the `messages.pot` file::

    $ ./manage pyenv.cmd pybabel extract -F babel.cfg \
              -o ./searx/translations/messages.pot searx/
    $ git diff ./searx/translations/messages.pot

----

hint from @dalf: f-string are not supported [1] but there is no error [2].

[1] python-babel/babel#594
[2] python-babel/babel#715

Closes: https://github.com/searxng/searxng/issues/3412
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/engines/yummly.py

@@ -68,7 +68,7 @@ def response(resp):
                 'title': result['display']['displayName'],
                 'content': content,
                 'img_src': img_src,
-                'metadata': f"{gettext('Language')}: {result['locale'].split('-')[0]}",
+                'metadata': gettext('Language') + f": {result['locale'].split('-')[0]}",
             }
         )