In a formal learning environment like a classroom, sure. Native language (or even another second language) is a crutch to associate concepts in the new language

In a formal learning environment like a classroom, sure. Native language (or even another second language) is a crutch to associate concepts in the new language. Informal environments without a base language, I'm not sure many (if any?) know how that feral person learns, but we do know it is possible for them to communicate using language. I think a lack of metalinguistic awareness plays a major role in language learning as the learner would have no way to reflect/process on the language. This skill is huge, and it's built in ages 6-8. This is the language skill that lets us discern literal and implied meaning from language (statements vs sarcasm), reflect on non-literal concepts (idioms and "love") and analyze and correct incoming communications that contain incorrect usage in word choice, grammar or pronunciation (e.g. If someone misspoke "He is...to the store...going," we infer the speaker likely means "He is going to the store"). Without a base language and meta linguistic skills, language learning would be done with very concrete utterances and gesturing which is consistent with immersion learning of a second language, but the learner's fluency will likely never progress to the point that a second language learner's fluency could reach.