Untranslated elements in non-fallback language appear for end user

Description

Form elements ends up being shown to the end user even if the elements haven't been translated to the site language nor any applicable fallback language.

Steps to reproduce:

1. Go into the edit view in CMS.
2. Select the site language (e.g., sv).
3. Create a new form.
4. Add an input text field and a submit button to the form.
5. Switch the site language (e.g. en).
6. Make sure that the language doesn't have fallback language set to the previous one.
7. Translate the form container to the current language.
8. Copy the form elements from the previous language branch; do not translate these elements.
9. Publish.
10. Drag the form into an applicable content area and publish.
11. View the page in the secondary language (e.g. sv).
12. Submit.

Expected:

Untranslated elements should not appear. Form should be empty.

Actual:

Elements appear with texts from their master language. And if a user submits the form, the associated values are not present in "Form Submissions" in edit mode.
In the log, a variation of the following line appears for each untranslated element:

ERROR EPiServer.Forms.Core.Internal.DataSubmissionService: Element 'xxx' not found or the Form has not been re-published after was changed.