"To which extent?" vs. "To what extent?"

Is there a difference in meaning between "to which extent" and "to what extent"? Are they used in different geographical areas?

The two phrases mean different things:

"To what extent" asks the question in general without any specific extents being presupposed beforehand.

Example: "To what extent would you be willing to help me move this weekend?

"To which extent" chooses between several pre-existing options/distinguishes between them.

Example: "You could just let me borrow your truck, or you could carry my couch up the stairs. To which extent would you be willing to help me?"

I think the confusion was created by non-native English speakers. In several other European languages (most of Slavic and Romance ones) the word translatable as "which" is used in this and many other situations where in English is used "what". So, for those speakers is just more natural to say "which" in this and other cases, since many are not aware that English "what" has a wider usage than "what" in other languages, i.e. that it is often used in situations where in other languages people use "which". I do not have any scholarly sources to support this statement, it is my opinion based on my own experience (since I am not a native speaker) and my teaching experience (teaching native and non-native English speakers)