I could not find an answer on their github. I think all messages and errors are in English on my system with PowerShell 7.1.4.
Although Get-UICulture, Get-Culture, $PSUICulture, $PSCulture are all fr-FR. I forced an update-help just in case to no avail.
Can you confirm that PowerShell 7 is not translated yet ? If so, is it in process ?