The key process of translating a page (or the new version of a modified page) consists in 2 distinct steps: 1 enable the translation ("mark for translation") 2 translate.
Bu why does
https://wiki.inventaire.io/w/index.php?title=Special:Translate&group=page-How+to+contribute&language=fr&action=page&filter=
(note the parameter &group=page-How+to+contribute) have a redirection to the generic
https://wiki.inventaire.io/w/index.php?title=Special:Translate&group=%21additions&language=fr&action=page&filter=
which is preventing from translating the "How to contribute" page, since no group of text to translate can be found on the translation page
I guess there is something to find out at the special translations page since How to contribute belongs to the last section "Pages in translation"