Entities are not validated when adding a new translation

Problem/Motivation

I encountered this issue while translating a node with a translatable link field. The translation contained an invalid URL for that link field. The new translation for the entity was however saved succesfully. When I visited the translation, an exception was thrown by the LinkFormatter which was trying to display a link for an invalid URL.

I then found out the Content Entity Source Plugin doesn't validate the new translation before saving the entity.

Proposed resolution

Validate the entity before saving a new translation.

Remaining tasks

(reviews needed, tests to be written or run, documentation to be written, etc.)

User interface changes

(New or changed features/functionality in the user interface, modules added or removed, changes to URL paths, changes to user interface text.)

API changes

(API changes/additions that would affect module, install profile, and theme developers, including examples of before/after code if appropriate.)

Data model changes

(Database or configuration data changes that would make stored data on an existing site incompatible with the site's updated codebase, including changes to hook_schema(), configuration schema or keys, or the expected format of stored data, etc.)

Original report by [username]

(Text of the original report, for legacy issues whose initial post was not the issue summary. Use rarely.)