TYPO3 Core

I have not fully understood, reading the description, why the L=0 param should be added to the previews in default language; could some use case or example be added?

Anyway, I'll write here what I found testing the latest 10.0.0-dev master:

Context menu page tree => view:
Always shows the default language pages; the issue could be solved with/after #46017 IMO

Page module, display mode columns, view button in doc header:
Still troublesome; only the default lnaguage is shown, regardless of the language selected with the language dropdown

Page module, display mode languages, view button in doc header:
Still troublesome; only the default lanuage is shown, regardless of the language selected with the language dropdown; Instead, the preview button under the pagetitle in each language column works correctly.
By the way: in display mode: languages the dropdown in docheader that allows to choose language could be omitted. Or not?

View module:
Seems to work correctly; both the preview inside the iframe and the button lead to the correct language variant.

Edit content in default language, button save & preview:
On 10.0.0-dev only "view" button is used; it seems to work correctly; the page with default language is shown

Edit content translation, button save & preview:
On 10.0.0-dev only "view" button is used; it seems to work correctly; the page with selected language is shown

I my case the default language in backend was EN (sys_language_uid=0), but the frontend would show another language (sys_language_uid>0) when no L-param was added.
To make sure the preview link leads to the language currently selected in BE, I think the L-Param should always be included.