For me, it has to be Korean (Hangul is easy to grasp) but the pronounciation, my god: it’s like I can’t understand a word being said when they speak (while that part is manageable in listening to spoken Japanese). Maybe it’s just me not having the “trained” ear on listening to Korean phonology in real time whilst learning the language.
For me, written French is what it sounded like centuries ago, instructional (vocal) French being an approximation of what it sounded like some years back, and modern, spoken French somewhat different yet again, particularly by region.
It’s like learning three languages in one, and I haven’t even gotten to the third one, yet.