Support UTF-8 encoded strings as value for :translate()

Without the patch the following JSON snippet results in a UnicodeDecodeError:

    {
        "_id:translate(onegov.policy.contentcreation)": "bruckenangebote",
        "_type": "ftw.topics.Topic",
        "title:translate(onegov.policy.contentcreation)": "Brückenangebote"
    }

Assumption is that the JSON file is encoded as UTF-8 (which seems a little better than ASCII).