Describe the bug
If you have a multi-lang setup and duplicate a page that only has a content file for the primary language, the newly created page will undesirably have content files for all the other languages.

To Reproduce
Steps to reproduce the behavior:

1. Assume you have a multi-lang setup with en (primary), fr, es
2. Go to an existing page that only has the primary en content filled in. (On disk, this page has no fr or es content files.)
3. Duplicate the page (if relevant, assume "copy files" is also enabled)
4. The newly created page will have files for en, fr, and es
5. (Interestingly, content meta files for images will not have extra versions.)

Expected behavior
I would expect it to only duplicate content files for languages that actually exist.

This current behavior is extremely undesirable and non-obvious.

Kirby Version
3.3.3 and 3.3.5-rc.1

Desktop (please complete the following information):

• OS: macOS
• Browser: Safari
• Version: 13.0.5