Unfortunately, you can't. The list of available languages is a system configuration option that affects the Voice Search app, but can also be used by other apps, and by design isn't intended to be updated that often.
Google assumes that a user sets the desired language and performs actions based on it, as one tends to use the native language to perform such actions, the list is supposed to contain as much available languages as possible.
From the recent post on the Google Blog announcing 13 additional languages for Voice Search, it's mentioned:
Note that you can only speak one language into the app at a time, and you may need to change your language settings to use one of these new languages.
The list itself is extensive as to allow developers to provide support according to their target users, and as mentioned, to allow users to select their own desired language.
A workaround would be to have a custom ROM limited to the desired languages.