To my mind assimilating Cantonese from Mandarin or vice versa is way easier than French <> Italian or Italian <> Spanish. Spanish <> Portuguese is an interesting contender.
Good luck pretending Mandarin and Cantonese are distinct languages while comparing German and French lol.
I say this as someone with a whole lot of Cantonese dna in my heritage before people get all up in arms. I've personally always figured the barriers to learning both Mandarin and Cantonese was cultural and there are plenty of people in Guangzhou who are perfectly bilingual