Hierarchical Select doesn't work if you disable English language

How to reproduce

1. Create a new site with two languages enabled, in my case: Catalan (ca) and English (en).
2. Add an hierarchical_select in the 'page' content type.
3. Create a new node (node/add/page). This should works.
4. Set Catalan (ca) the default language and disable English (en) in /admin/settings/language/overview
5. Create a new node (node/add/page) and use the hierarchical_select combo. This should fail with a "Received an invalid response from the server" error message.

This is failing because the AJAX system is trying recover the data from _YOUR_SITE_/ca/hierarchical_select_json, and this is returning a 404 page not found error.

If you only have one language enabled the correct URL is _YOUR_SITE_/hierarchical_select_json, not _YOUR_SITE_/_LANGUAGE_CODE_/hierarchical_select_json, this second call it's only correct with a multilingual site.

6. If you return to /admin/settings/language/overview and then enable the English (en) language, hierarchical_select works again.

So, you need to enable English language, although not using, to ensure that the hierarchical_select_json calls are working.