Problem/Motivation
See #3110954: Paragraphs field translatability checked by default when reusing fields.
I think this is a bug/inconsistency in Field UI.
New fields added in the UI default to not-translatable, per #2143291: Clarify handling of field translatability that is done so explicitly, with a comment.
However, when re-using an existing storage then that line is missing and those fields are then translatable by default.
This is especially awkward when translation is not enabled for a bundle, as then the checkbox is disabled and it can't be changed.
This was last touched by #1963340: Change field UI so that adding a field is a separate task, didn't check if that introduced this problem or just moved things around.
This has been like this for a long time, but I think this is a bug that we should fix.
Proposed resolution
Also set translatable => FALSE when creating a field for an existing field storage.
Remaining tasks
Tests