The easiest way to learn a language is simply to put a child in that environment, an environment where they have no other option but to learn. Granted, it's can be immensely stressful, but having been in a similar situation, I can attest that it works.

I grew up in Canada my whole life, but was taken to Taiwan towards the end of my elementary years and enrolled in the normal school system there without so much as knowing the alphabet. Spending 3.5 years there allowed me to pick up the language fluently, at a fluent, accent-less level (reading, writing, speaking, listening). While there I also learned "Taiwanese" at a fluent, accent-less level while visiting produce and night markets.

Now when I interact with Mandarin speakers in Mandarin, they assume I grew up in Asia, and vice versa with English.

On the flip side, many Asian friends who grew up here attended Saturday Chinese schools, and although it helps allow you to communicate on a basic level, most hate learning Mandarin, and thus fight it. Speak with classmates in English the second the teacher isn't hounding them, speak in English during breaks, etc. Many end up not even being capable of conversing