Support translated labels in featuretree

This PR allows the possibility to have translated labels for search results in the feature tree. It looks for `label_xx` entries on the results, and takes the one corresponding to the current language. If it's not found in the results, it takes the `label` attribute, which should always be present.

See also https://github.com/geoadmin/service-sphinxsearch/pull/146

Please review/merge quickly as we want this in next weeks deploy.

(Note: this could be extended to other locations, mainly the search)