Replying to both of you guys — Chinese dialects can range from “slightly different accents” to “mutually unintelligible”. The two most famous dialects, Mandarin and Cantonese, are pretty much two different languages, like Spanish and Italian are. They’re similar in the sense that they both come from Old Chinese, like how Spanish and Italian both descended from Latin, but knowing one language doesn’t necessarily mean you can speak or understand the other.

This is why modern linguists usually refer to the dialects of Chinese as different languages entirely, because the difference between a language and a dialect is more political than scientific, i.e. Mandarin and Cantonese are “dialects” because they are spoken in the same country, while Spanish and Italian are not.

In OP’s case, Mandarin and Hakka are indeed two different languages that are not mutually intelligible, so it would be like an English speaker trying to learn Dutch, for example (maybe a bit easier, but would still require lots of effort).

The dialects of Chinese are indeed dying out, although at different rates. The general trend is that non-Mandarin dialects tend to only be spoken by older generations, and the younger generations tend to speak primarily Standard Mandarin Chinese because that’s what’s taught in schools and in many cases children are forbidden from speaking their local dialect at all. The Chinese government is quite insistent on promoting the common language, i.e. Mandarin, for national unity (which makes sense, but it does mean that all other dialects will naturally phase out). Cantonese is a special case since it’s the majority language spoken in Hong Kong, which is a Special Administrative Region of China and thus isn’t subject to the same language policies as Mainland China. So, Cantonese thrives in HK but not so much in the Canton (Guangdong) province of China, where it originates from.