This is a simple one: you used the wrong link to download en.json 
The link shown on the other thread gets you the file embedded in html. But you need the raw file found on the link I stated above: https://raw.githubusercontent.com/mattermost/platform/master/i18n/en.json .
Hint: Don't forget to su mattermost or sudo -u mattermost or you won't get anywhere - the postgres db won't open.