The language attribute is present in all layouts since commit 025923ac4,
so there's no need to use the language selector locale. Besides, it
wouldn't work if there's only one locale and the language selector isn't
shown.
Patch extracted from here the comments on turbolinks issue 253 and
converted to vanilla javascript.
The hide action over datepickers ensures us that opened datepickers
will be closed before leving the page. Previously if you open any
datepicker and then move to previous page you will keep seeing the
datepicker in the restored page.