Fixes for adding language filter to sync-ingredients management command.

[scrapcode]

Proposed Changes

• Fixes as requested in code review for PR Add language filter to sync-ingredients management command. #1875
• Allows all languages to be passed at once
• Allow user to specify a language when syncing ingredients
• example: python manage.py sync-ingredients --languages=en,fr,es

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
  • Seperated test for --languages feature to ensure correct URL is being called
• Added yourself to AUTHORS.rst

Other questions

• Do users need to run some commmands in their local instances due to this PR
  (e.g. database migration)? No

[rolandgeider]

Thanks for the pr, will test it once I get home, but from a first glance it looks good

[scrapcode]

Removed duplicate functionality I failed to remove in initial merge conflict.

[rolandgeider]

merged, thanks!