Page should serve content in default culture in case it isn't translated into target culture

[lukas-xb]

Motivation

This looks like a common use case:
Not all of the website content might exist in all culture variants. In this case, developers might want to display content in default culture. The training website in the Essentials module should show how to achieve this behavior.

Example submitted by a customer:
For example, I have 5 doctors in my clinic, all of them having an English description but only one of them has been translated into Spanish.
In my live site, in the English culture, I see 5 doctors but when I switch to Spanish, I see only 1 doctor while I would like to see the 4 others even if their associated information is in English.

Proposed solution

Ideally, this behavior will be implemented globally, e.g., by calling CombineWithDefaultCulture() in DocumentQueryService.