Consciousness without language It seems that consciousness can only be investigated through self-report, i.e. language/communication of some kind. Is it believed that consciousness could nonetheless exist without language?
Yes, it is assumed that non-human animals have varying degrees of consciousness - but this seems somewhat unverifiable without our capacity to ask them.
Conversely, has there ever been a human being to grow up without language for years and then learn a language sufficiently well to be questioned regarding their subjective experience prior to learning the language?
Language is a means of expression and communication. It serves as a medium of expression. Consciousness is independent of language. One (human or any animal) can be conscious however to report and verify, as you said, we need a language or any medium of communication.
Consciousness could safely considered an axiom in psychology.
For the second question, you have bumped on to one of the canonical pairs. How do we explain nothing? Ideally we cannot. The same is with consciousness. It is a fundamental thing. A formal verification might require a revisit to the fundamentals of philosophy and psychology.
Reference:
Thought without language: Thought without awareness? - L.Weiskrantz