In the REST API, the languages parameter to filter fields on a given language is not working as expect on the load content and load version resources. (not tested but most likely the same issue happens in the Public API)

For content with the always available flag, the fields in the main language are always included.

Steps to reproduce:

1. Create a content with the always available flag in eng-GB (for instance a Folder)
2. Translate it into French (or whatever)
3. Run the following command

   curl -H 'Accept: application/vnd.ez.api.Content+json' -H 'Cookie: eZSESSID=t6tcfvvpr0g671bppaf9h7glk7' 'http://ez.loc/api/ezp/v2/content/objects/1?languages=fre-FR'
   

it should only return the fields in fre-FR, it actually returns both eng-GB and fre-FR.
It seems it's related to https://github.com/ezsystems/ezpublish-kernel/blob/master/eZ/Publish/Core/Repository/ContentService.php#L377-L396