Bug 32251: Add a fallback for when language cookie was removed

Bug 32251: Add a fallback for when language cookie was removed

Since we remove cookies after logout now, the language cookie
might not be there yet. It is friendlier to add the template
language fallback here now too before switching to 'default' in
the Additional contents (AC)-context.

Note: The cookie will not be removed when you added it to the list
in koha-conf.xml.

Note2: This only pertains to the use of opac-page with code param.

Test plan:
[1] Create a CMS page with AC. Add two languages (say en, nl-NL).
    Add different text to default and both languages.
[2] Remove the KohaOPACLanguage cookie from your browser (via
    dev console).
[3] Without this patch, copy the Current language URL from the AC
    overview page. (Note that the Default URL always refers you to
    'default'.)
[4] Go to that page with the selected (Current..) URL. Observe that
    you still see the 'default' text.
[5] Remove the cookie again if it might be there already.
[6] With this patch, observe that you get the language of the
    interface now on the CMS page using the same URL.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Pascal <pascal.uphaus@gwdg.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>