Update main.yml

add 'EDXAPP_LANGUAGE_CODE' and default it to 'en'
add 'LANGUAGE_CODE' and default it to   $EDXAPP_LANG
Update main.yml

define a new var called "EDXAPP_LANGUAGE_CODE " and default to "en"
add "LANGUAGE_CODE: $EDXAPP_LANGUAGE_CODE "
Merge pull request #894 from fmyzjs/patch-7

Update main.yml