Pages copy: create tags in correct language.

During page copy, the tags were not created in the target language.
This change sets the target language when saving tags, and also makes
sure that BackendTagsModel::getURL() uses the correct language, so that
the slug for the tag does not have any unnecessary '-2'-like suffix.
Merge pull request #578 from forkcms/fix-pages-copy

Pages copy: create tags in correct language. (REVIEW FIRST)