TYPO3 Core

I think that this has been at least partially solved in version 9.2.0-dev (latest master).

I tried the following test

Prerequisites¶

1) TYPO3 with default language (ITALIAN) and two additional languages (ENGLISH, FRENCH)
2) BE group: Editors, give full access in ACL
3) Editor user (no admin), limit it to languages ENGLISH, FRENCH

Test¶

1) create a page and some content
2) translate the pagein in French, English and translate the content (Translate mode)
3) give the page group permissions "editors"

Results¶

1) in Page view > Languages you see the three languages, but the edit pencil is only with french, english
2) in Page view > columns you don't see any pencil to edit pages (!) even in FRENCH, ENGLISH - you can only edit content in FRENCH and ENGLISH
3) In List View you can't see any pencil to edit the pages(even in FRENCH and ENGLISH)
4) about the context menu that opens when clicking on the page icon:
4.1) in ITALIAN it shows all the actions, even "new" and "edit"
4.1.1) when clicking on "edit", an error is shown (operation not permitted)
4.1.2) when clicking on "new" it creates a new language with language_uid=1
4.2) in FRENCH all the actions are visibile
4.3) In ENGLISH only "show", "info", "more options", "history/undo" are visible. I don't know why (the language has id=2)

There are inconsistencies in the interface but at least it seems that the editing of the original language is forbidden.

I haven't done further tests; do you think that there are different scenarios to test?