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.