Multi-language wiki: single language Sandbox pages are shown as "without language"

Steps to reproduce:

• set wiki to multi-language
• search for "Sandbox"
• open the "Language" facet and select only for pages with "No Language"

This creates three search results: Sandbox.TestPage1,2,3

I guess these pages should have an explicit defaultLanguage=en (unlike most pages in platform, who are indeed not in any Language, but like the most of the page in xwiki-platform-help-ui).
What I am not so sure about is if that change will create lots of conflicts when upgrading the content ... in that case it might not worth the change.