Its language is a language which the soul alone understands, but which the soul can never translate.

Bennett is describing an experience—often art, music, or profound feeling—that is immediately intelligible at an inward level yet resists paraphrase. The “soul” can recognize the meaning directly, but the moment one tries to render it into ordinary language it slips away, because discursive speech is too blunt for the nuance of inner life. The line points to a modern tension between lived experience and expression: we may be certain we have understood something essential, while also knowing that any explanation will be a reduction. It also implies a kind of humility about criticism and self-reporting: some truths are apprehended, not translated.