Translation API fixes

Summary:
- Fix limiting translations from api by parent id
- Handle string locale param to get a translation

Test Plan:
`./bin/behat -c application/tests/behat.yml application/tests/features/api.posts.feature`
`./bin/behat -c application/tests/behat.yml application/tests/features/posts/api.translations.feature`

Reviewers: eyedol, shadowhand

Reviewed By: shadowhand

Maniphest Tasks: T438

Differential Revision: https://phabricator.ushahidi.com/D278