tl;dr I need a better hobby because I waste way too much of my life on learning useless things
Really? You don't say. Because I'll put my OCD encyclopedic knowledge of the last 100 years or so of Indochinese history as infinitely less useful. But anyway.
Here's my story. Spanish primary language, but so thoroughly obliterated by English. I also speak passable German. Took two years of Latin in high school, and two years of German at university which improved my grammar and spelling greatly. I've taught ESL classes at community colleges, and find language study fascinating. So this all comes out kinda scattershot.
First of all, I don't care what Italians say, it is ao similar to Spanish that I understand the majority of what I read or hear. Can't speak it at all, probably couldn't even ask directions. Portuguese less so, but still can decipher most of what I read, as long as it's things like signs, menus, etc. I've picked up a smattering of French just by translating French quotes in books and from culinary studies. I can identify writing in Romanian, but that's it.
German is pretty damned close to Yiddish so long as I'm hearing it, and have been able to converse in rudimentary, simple sentences with a Yiddish speaker. I can recognize Dutch, but not much more. I can't tell the difference between Danish, Norwegian and even less Swedish. Could probably decipher street signs and the like. Afrikaans is another world. Some words are just like pidgeon German, but hearing it spoken leaves my brain spinning.
I can identify Finnish when I see it, as well as Hungarian. Completely useless with any Slavic language. Polish I can recognize, but Czech, Bulgarian, Serbo Croat, Russian, I can literally identify about four letters in Cyrillic. Arabic, African, Oriental languages, useless. I tried learning a little Vietnamese, but had to give up as it is a language all about inflection, and I wouldn't get it without somebody to practice.
As far as difficulty to learn, I'd put it at English, Spanish, German then Latin, hardest to simplest. I've never, ever, figured out how to properly place accents in Spanish. Many people have told me it's very simple, but those were always the last words I actually understood once they started the rules. My mother has been a school teacher my whole life, and she gave it up as a lost cause after about two tries. And I'm deliberately leaving out the issue of proper Castillian versus common Spanish, and to hell with Basque, Valencian and other forms. I can't read Garcia Llorca or Cervantea without my Real Academia de Espanol (Royal Academy of Spanish) dictionary.
As has been mentioned before, the primary stumbling block I've heard from students is just too many spelling variants, owing to the great numbet of languages English draw words from. Once, in order to help my students understand better the movements of people that have influenced English, I drew a pretty decent map of Europe and the Mediterranean on the chalkboard and proceeded to go from ancient Greece to the late middle ages. My students were flabbergasted that I drew such a detailed map of Europe freehand, and heard nothing else I said after that. Oh well. The different spellings completely negated any advantage our simple grammar format afforded. Without the stability of learning fixed declensions and conjugation, you essentially have to learn every word as if it had its own rule.
Think about it. The verbs drive and dive. I drive, I drove. I dive, I dove, or is it dived? He drove here and he dived in. And that's just one example. Trust me, that drives them bat shit crazy. Lack of uniform rules fucks with your stress levels in a big way.
So, until I die, I'm going to still love learning about languages. I'm going to have to suck it up and try at least one Oriental language in my life. And it sure as shit won't be Chinese. I'm still pretty pissed all the words I grew up learning like Peking duck and Mao Tse-tung and Yangtze just got tossed out the fucking window. Tough shit, you have to spell them the way we tell you to now, European barbarian.