Hello.
After installing opensuse how to have ( in KDE or init 3 )Keyboard layout french
All system messages in English ( error, warning, info …)
All formats as fr_FR (numbers, time, currency, measurement)
And what option to use for collation and sorting ?
Not sure how helpful this might be but have you checked YaST>Language for the language setting and YaST>Software management for French language packages that might have inadvertently been missed?
Unless you have a special reason for using a non-UTF-8 collation, stick with UTF-8.