multilingual Image fields - error with same default/lang

Problem/Motivation

multilingual field_image (classic image field), langs: default, english, french

want to have:
default: path_img + alt: grp_en
english: path_img + alt: grp_en
french: path_img + alt: grp_fr
this results in file cant be referenced.

also crashes when 'default' + 'english' are set

Only when I have 'default' + 'french', goes well.

So it rejects default=english and translation-alt=english.
Accepts default=english, translation-alt=french

When performing those mappings on eg. link and text fields, I can set the 3: default, english & french. works fine