From a Seo stand point I assume there is no disadvantage of having non-English websites on an English directory? What is the disadvantage? I guess for the directory user/searcher it may not "look" as nice?
A directory is meant to be searched by users so of yours is a English one how could they understand chinese or Russian languages? Also how it could benefit from the SEO point of view
There is no SEO standpoint. The directory aim and primary purpose is to give surfers a quality surfable directory. Having a non-english website in a English directory belongs to the category [Regional or World] if there is. There is no disadvantage unless its an illegal site. I would prefer a non english Language School who have submitted to one of my directories rather than a viagra site in English. It always look nice for directory owners to give service to its surfers. The main objective. Otherwise if its SEO or just simply making a quick buck, then their doomed.
In all of my directories I will always have room for non-english site. The site will still apply with the TOS just like any other site.