Roomshare Japan is Japan's largest roommate classifieds (based on Alexa and Google ad tool). Recently, more and more Japanese became to know this comparably new lifestyle and most users are Japanese using Japanese language. However, many of long time users since site open (1999) have experienced to share with foreigners.
Posters who are okay to be contacted in English prepare English title/text. If he/she only post in Japanese, it is possible that they do not, or can not read and reply your English inquiry.
Yuki Aki, the founder, published a guide book on how to live in roomshare, which was the first one in Japanese language.