Root locale as the first valid token from locales if no "default" included

[pbaumard]

This closes #1226

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[michael-o]

I went through the PR, the old code and the Doxia Sitetools and refusing this for the following reasons:

• Although you are right, the docs aren't correct, I need to fix them, the previous behavior modified the entire JVM's locale which can have negative side effects in a MT build, we removed this change
• Your change ignores to modify log messages regarding the default locale
• You completely forgot to align with Doxia Sitetools because its behavior relies on SiteTool#DEFAULT_LOCALE throughout. We need a complete solution covering Doxia Sitetools and Maven Site Plugin.

I am currently thinking about something like this to give you to modify the Doxia Sitetools default locale:

 Locale DEFAULT_LOCALE = getDefaultLocale();

where you can supply mvn -Dmaven.doxiaSitetools.defaultLocale (or similar). Maybe there is a more elegant way to do this.

@kwin What do you think about this?

[michael-o]

Draft solution: apache/maven-doxia-sitetools#623