HS i18n support

Hierarchical select doesn't play well with i18n on the node edit forms. i18n sets a couple of variables when on the node edit form, but when the form is called through ajax in hierarchical_select_json, these variables don't get set, because the arg(0) isn't 'node'. I originally tried restoring the arg's #486482: Restore path, but discovered that solution had other side-effects.

To reproduce the problem, you need a content type with a translatable taxonomy that uses hierarchical select. The taxonomy should be translated using terms (the vocabulary language is empty, and the taxonomy language is set). When you try to node/add this form, the hierarchical select terms are originally set to the right language. But when you pull down the hs term to get the second level terms, the ajax callback now has the wrong language, so it populates the top level now with the wrong language terms.

The attached solution is to save the i18n mode and params in the cache when the form is built and saved (in hierarchical_select_after_build), and then to restore these i18n variables when the form is rebuilt in hierarchical_select_json. I don't like this solution because it has built in specific knowledge about i18n. But it's the simplest solution I've been able to find.