About the workaround. After creating the English language branch I can select the page as ThankYouForPosting-page, after which I can destroy the English branch and the page will still remain selected in the XForm. But I find this process a bit too cumbersome, though.
Bug (#118178: Can't select XFrom after-post-page that's not in site's default language) has been fixed and will be released soon.
A fix for above bug was released in EPiServer.CMS.Core 7.16.0 (update 41)
Looks like it's impossible to select XForm "Page shown after the form has been sent"-page that's not created in site's default language. For example, if your site has default language English, and you create a Norwegian page with XForm property and also a Norwegian ThankYouForPosting-page, it's not possible to select the Norwegian ThankYouForPosting-page in the page tree dialog.
The page tree menu displays the Norwegian ThankYouForPosting-page with a disclaimer "Page is missing for the current language and will not be displayed", and it's not selectable, though the language context definitely is Norwegian.
One workaround is to create an English language branch for the ThankYouForPosting-page, but that will cause issues if the page should only exist in Norwegian.
Has anyone else encountered this problem and conquered it?