What happened?

The fr-FR language pack (administrator and site) sets firstDay to 0 (Sunday) in
langmetadata.xml, instead of 1 (Monday).

This value is read by Joomla\CMS\Language\LanguageHelper::getMetadata() /
Language::getFirstDay(), and used by the core Calendar field
(libraries/src/Form/Field/CalendarField.php, data-firstday attribute) to determine
the first day of the week shown in every date picker across the backend and frontend.

Steps to reproduce:

1. Install/use Joomla with the fr-FR language pack.
2. Open any admin or site page containing a calendar/date field (e.g. Users > filter by
   registration date, or any custom component form using a type="calendar" field).
3. Open the date picker widget.

Version

6.1

Expected result

For the fr-FR (French) locale, the week should start on Monday, matching both the
ISO 8601 standard and standard French convention/UX. This also matches
administrator/language/fr-FR/langmetadata.xml and language/fr-FR/langmetadata.xml
from earlier Joomla 3.x-era fr-FR packages, which correctly had <firstDay>1</firstDay>.

Actual result

The date picker's week starts on Sunday, because both
administrator/language/fr-FR/langmetadata.xml and language/fr-FR/langmetadata.xml
currently contain:

<firstDay>0</firstDay>

it should be:

<firstDay>1</firstDay>

System Information

6.1.3

Additional Comments

No response