I don't think so. The code to blame is found somewhere starting at line 22 in utils/i18n.go and tells that the language is has not to be defined in config.json because all language files from the translations directory, which is incidentally now always the current directory 
, are used. Unfortunately all *.json files residing there are loaded, even the files which aren't language files like config.json and the i18n library gets mad about that.
The message no language found in "config.json" does not mean that a language definition is missing in mattermost's config.json file but that the config.json found in the translations files directory isn't a translation file at all
tl;dr: The en.json must be the only .json file in your current directory while you are running the mattermost cli command