doc(translatable): Improve the LocaleSubscriber implementation

[damienalexandre]

Also fix some English / improve the flow of the document.

At the start all I wanted to replace was the locale parsing / setLocale on the Request thing to use modern Symfony:

$request->setLocale(
    $request->getPreferredLanguage($this->availableLocales)
);
$request->attributes->set('_vary_by_language', true);

[damienalexandre]

Gentle ping about this pull request, is there anything I can do to help you review it?

Cheers