I have installed the plug-in French Pack, but when I restart the SonarQube server, I have no changes, my SonarQube is always in English.
How passing all my SonarQube in French ?
If I create my own SonarQube widget, if I use the plug-in French Pack, automatically the widget is translate or not ?? If not how to do to translate a widget ???
Assuming you've chosen French localization in your browser, what you're (not) seeing is a product of the fact that the French localization pack hasn't been released in a very long time.
To answer the second part of your question, it is possible to localize a plugin by providing the correct localization files, e.g. org/sonar/l10n/mysonarplugin_fr.properties. That's documented here.