Restrict Wikisource translator to en.wikisource.org

The HTML nodes used by this translator are currently only used by en.wikisource.org, causing Zotero failues on other language based Wikisources.

Example of page where the failure is reproducible: https://fr.wikisource.org/wiki/Lettre_%C3%A0_M._Dacier_relative_%C3%A0_l%E2%80%99alphabet_des_hi%C3%A9roglyphes_phon%C3%A9tiques