Hello,
Some background information: WAMPP 3.3.0 64-bit on Windows 10, which involves Apache 2.4.54.2, PHP 7.4.33, MySQL 8.0.31, Firefox 109.0.
After checking on the server, it appears that HTTP_ACCEPT_LANGUAGE fr,fr-FR;q=0.8,en-US;q=0.5,en;q=0.3.
Moreover, as mentioned, it seems that Zenphoto's translation feature is partially working since on the login page, all the strings translated into French in the appropriate fields are available when activating in Options > General > the "Multi-Langual" checkbox. The only remaining problem is when an authenticated user tries to upload a photo, then the interface text appears in English (even if English is not a selected language, let alone a default language).
Is the English language (default) hard-coded or could it be overwritten by overwriting the file with another language file?
Thank you to all those who are working to find a solution to this problem.