But if you consider the reality, it is still English being the easiest language to learn for most Cantonese speakers for reasons of school curriculum, exposure to the English media and entertainment, usefulness, etc. haha

Being a second speaker of Cantonese and Japanese, I can tell you they are indeed quite similar in terms of shared vocabulary (when written), but that's it. The grammar is totally different: SVO vs. SOV; no verb conjugation at all vs. conjugating tenses, aspects, honorifics...; different system of demonstrative pronouns; etc. One interesting example would be in Cantonese one may say 玻璃破左 (the glass broke) but in Japanese ガラスが割れている (the glass is breaking) when they want to say 'the glass is broken'

The pronunciation is also very different. Cantonese has no voiced sound and distinguishes consonant by aspiration, while Japanese the opposite. Cantonese has 8 basic vowels but Japanese 5 only. Tones would change the meaning of a word in Cantonese but not in Japanese, while long/short vowels make a big difference in Japanese but not in Cantonese. The length of nasal or glottal ending of a syllable is also different.

Those shared vocabularies also bear only some similarity to a certain extent when spoken. There are mostly rules behind them, just like you can trace many English words back to its Latin origin, but some words are just hopeless. For example, Cantonese dropped all the voiced/unvoiced difference in Middle Chinese, so 嚴 and 鹽 sounds the same in Cantonese (yim) but different in Japanese (gen/en), or like 文 and 民 (man vs. bun/min). Another example would be Japanese lost all the -p glottal ending, so 九 and 急 are the same in Japanese (kyuu) but different in Cantonese (kau/kap)