I have no idea how big a job it would be to just inactivate this fixedI'm sure the programming effort is not the issue.
No doubt Mark is concerned with the potential for abuse, since by design Invitations can be sent to addresses that are not already a part of your group. If the entire message content were written by the group owner/mod then the invitation feature would be a very attractive tool for spammers and scammers of all sorts.
So the boilerplate is there to make the Invitations less attractive to spammers, and to give the recipient a clear understanding of what the message is about.
Obviously, it doesn't serve the latter function for those who don't speak English. But the solution to that I'm sure is more likely to come with some overall effort to provide language-specific site text.