1. in the DNS manager for your domain you can create additional host names by creating DNS a records. I would make the default language site www.somedomain.com and for language sites it is probably better to use folder based child sites like www.somedomain.com/fr/ and use related sites mode then if users sign into the main site they don't have to sign in again for a child language site whereas if you use host name based sites like fr.somedomain.com then the user will have a different authentication cookie for each site. Note that I would not use www.fr.somedoamin.com, but just fr.somedomain.com. So you create the host name in dns for fr.somedomain.com and you point it to the same ip address as www.somedomain.com, then in IIS you add a host header for fr.somedomain.com
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2. should be ok but again I would not use www.fr.somedomain.com, I would use www.somedomain.com and if I decided to use host name child sites instead of folders then I would make the second site use fr.somedomain.com not www.fr.somedomain.com, shorter is better.
3. no there is no sharing of content feature instances across sites. in related sites mode it can share users and roles but no content instances can be shared across sites. So if you are talking about the Shared Files feature no it will not be possible to use the same one in both sites. If you are just building links to files in the file system then I guess you could use the same links in different sites.
Hope it helps,