How to index two different sub-domains in Google


I have one domain and one subdomain hosted by Dreamhost.
The sub-domain is intended to contain the same site, but in a different language (Japanese).

I noticed however that in Google only the root domain is listed, and each search query with a Japanese keyword will have the english site as a result.
How do I tell Google that the sub-domain and the root domain are in fact two different sites???