When accessing Moodle.org, the language selection menu has one wrong 'Español - Mexico (es_mx_old)' language pack instead of the correct 'Español - México (es)' language pack.
There is also a non existing 'English (es_mx)' language pack in the drop down menu.
Please see Moodle Doc about Why_do_I_have_a_language_pack_named_xx_old.27_that_I_never_installed.
Thanks Germán for highlighting the problem. It seems that the update lang packs scheduled task had got stuck. Thanks to David Mudrak for fixing things by updating the language packs from the command line.
Just noting David's explanation for 'English (es_mx)' in the languages drop-down menu:
The reason why the language menu shows it is because moodledata/lang/es_mx/ is incomplete and its langconfig.php is missing so the translation of the string "thislanguage" (name of it) falls back to the default English.