I don't know what task could possibly exist for which there are not significantly better and worse languages. Even today's "list of practical projects that anyone can solve in any programming language" [1] I can see at a casual glance that any of them that are not simply trivial will have better and worse languages. A definition of "lanugage agnostic" that either covers no tasks, or covers only very trivial tasks, isn't very useful. (Although in the latter case one can attain some insight by pondering why only such trivial tasks might be considered "lanugage agnostic".)
I'm not sure what any of these logical contortions have to do with SICP being 'language agnostic'. It isn't. The first section of the first chapter talks about why a Lisp dialect is used. This is the first figure.
[1]: https://news.ycombinator.com/item?id=14481941