In 10.0 since 52f268d0 some parts such as the date/datetime widget are
adapted to the current locale and not just displayed in english.
But the dates in the slides module are always in english because they
don't wait for the locale to be loaded.
To wait for the locale, this commit add a wait for WebsiteRoot.instance
ensuring that locale and other WebsiteRoot widgets are ready.
This may have been preferred to create a dummy child widget to
WebsiteRoot, this way only WebsiteRoot would need to be ready and this
would have been more extensible but this is a bigger change.
note: this fix is for 11.0, for previous version the fix is ef8cd11a6b
opw-804555
closes#22241
Nicolas Lempereur
06ebd7d6e5