Changing translation of saved draft causes a problem with publishing it. Form validation fails, even though all required fields are filled in.

Steps to reproduce
1. Create new language (e.g. pl-PL) in PlatformUI and add it to ezplatform.yml
2. Create new content (e.g. an article) in the default language (in this case en-GB).
3. Save draft.
4. Change translation of this draft.
5. Hit "Save" or "Publish".
6. Note that there is an error message:

Value for required field definition 'intro' with language 'pl-PL' is empty

Note: there are inconsistencies in POST request:

VersionUpdate {initialLanguageCode: "pl-PL"}
field {languageCode:"eng-GB"}