Hmm, normally the admins need not to have access to very personal settings of dedicated users. Thatswhy the language option is resticted to the profile page and not visible at the user page the admin uses to maintain users.
But you are right, if at multisite installation each blog has got an other default language configured, than this should be respected as default for the user. I have to check, if this is working and will make it working if not yet possible.
The only questionable thing is: If a user is member of more than one blog this user will than get in case of profile mode the last saved language at each of them regardless what lang the current blog will have. To circumnvent this, you have than only the chance to enable the lang admin bar switcher to allow users to switch easy to another lang.
In my opinion it makes not sence to store per blog a user lang but it makes sence to respect the sub blogs configured lang in first place.