That’s true, English and German are really not mutually intelligible, though the relations between the languages help to learn the respective other language. The only other language to German where a restricted mutual intelligibility exists is Dutch. As far as I can assess it, those two are similar to Italian and Spanish in that regard.
I worked briefly, with a Russian and a Pole. Training was given in English: the Russian had better English and helped out the Pole. I was impressed, I commented that I didn’t realize that he (the Russian) could speak Polish. He gave me a look, and said:
“Maybe in Warsaw and Moscow they speak different languages, but in the country, on the border, it’s the same language.”
Anecdotes where the common element is my father. His first language was Portuguese even though he was born in the U.S. He’s fluent but hasn’t spoken it regularly for decades. When I was moving out of my old apartment, he wanted to ask the Hispanic maintenance guy if we could dump some stuff on the loading dock so he spoke Portuguese as slowly and clearly as he could and hoped the maintenance guy would catch on. There were a lot of hand gestures. The maintenance guy just looked confused and didn’t seem to understand a word my dad was saying but at the end, when my dad asked, the maintenance guy said, “si, ok,” smiled and nodded. I have no idea if he was just trying to shut my dad up or if he really understood.
Also once when my wife and I took my dad to Portugal, we met up with my cousin at his convenience store. My cousin spent over 40 years living in Venezuela. My wife doesn’t speak Portuguese but she does speak Spanish reasonably well, so she spoke to my cousin in Spanish. My dad listened to the whole conversation and seemed to understand but didn’t say much. At the end, my dad said to my wife, “I didn’t know you spoke Portuguese.” She responded, “I don’t, we were speaking in Spanish.” My dad said, “oh, that’s why I couldn’t understand either of you!”
As a semi-fluent speaker of Spanish, I can just about understand the basic gist of Italian in a formal setting - a TV announcer or academic speech for instance. Hearing ordinary people speak in conversation is far more difficult. Indeed, my understanding of Spanish can be wobbly when it is in a less familiar dialect.
I find Italian to be easier to understand than Portuguese - whether European or Brazilian. The phonetics of Portuguese seem more distant from Spanish than Italian - even if the languages have closer geographic and historical ties.
Jorge Luis Borges and Anthony Burgess not only shared a last name (sort of) but a love for Old English. They would hold Anglo-Saxon conversations at Argentinian embassy parties to evade the ears of the junta regime’s spies.
The Italian-Spanish blend spoken as the local vernacular in Buenos Aires and Montevideo is called Portuñol.
I read an anecdote about an old Jewish lady in Sarajevo who overhead a diplomat from Spain speaking Spanish and thought he was a rabbi. The Jews of Sarajevo came there from Spain in Ottoman times and brought the Ladino language with them. Over centuries of no contact with Spain, they’d come to think of Spanish as an exclusively Jewish language.
I agree with this. For me the most difficult thing about understanding Portuguese is the cadence. Portuguese, like Italian, is spoken in a lilting, musical fashion, with a rising and falling intonation. To my ear, Spanish is spoken much more at an even level. I liken Portuguese to Spanish as spoken by a drunken Italian.
Native Spanish speaker here, half German, works as an interpreter from various languages, including Italian, into Spanish and German (what I consider my mother tongues, though what a native speaker is or the concept of bilingual can be a thorny debate among linguists). I also understand French and have a Catalan family. Italian is the last working language I learned, and starting from Spanish, with the help of Catalan and with the French crutch I managed to be able to work professionally from Italian into Spanish in just two years. I don’t speak it very well, I only have to understand it. Reading is easy. I just learned the differences, phonetics (as opposite to Portugese, which has a slavic sounding phonetic, with lots of sh and sch and tsch sounds completely unkown in Spanish and Italian and seemingly no vowels) is no problem.
Accents can throw me completely off, though. Italian has not been normalized from a central authority for too long, RAI (the state TV) was probably the main unifyer of accents, but only since the 50’s i guess. Sicilian, Sardinian, Calabrian accents seem to me more distant from Italian than Italian is from Spanish. Perhaps an Italian would have a similar feeling with the various South American variants of Spanish.
TL;DR: Spanish and Italian are very similar, when spoken slowly and clearly (not a natural for speakers of both languages, but doable with good will) more 90% mutual comprehension is easy.
I am bilingual in French, with years living in a bi-lingual Spanish-English household.
This is my experience, but reversed. I can understand Italian, but when I try to go beyond simple phrases I usually end up substituting French or Spanish words. Catalan is absolutely unintelligible to me.
This is absolutely true. 
I’m surprised how much of French, Spanish and Italian are understandable to a monolingual American. I’ve traveled around Europe and recognized a lot of words and managed to communicate.
I’d like to personally thank everyone who ever stole a word from France, Spain or Italy, and slipped it into the English language. And there were a lot of those…
tl;dr: pretty good, at least, Mexican Spanish and Sicilian Italian.
I have an actual example. I’m pretty sure that if you searched, you might find my story somewhere here.
Several years ago when I was married to my first wife, whom I’ll refer to as my wife, as this is past tense, I was living in Mississauga, Ontario. My wife, from Mexico, barely spoke English, so we enrolled her in an English school in Streetsville, where she made many friends.
Some long weekend I ended up with the prototype Ford Edge, and we decided to go canoeing at Algonquin Provincial Park, and invite her school friends. Our entire complement is me (American, can speak Spanish), my wife (Mexican from Mexico, no English), a male Italian lawyer from Sicily, a Columbian who lived in Germany who needed to learn English but spoke fluent German, and girl from Lichtenstein who only spoke German.
Clearly, the dominant language was going to be Spanish, and we all communicated pretty damned well. The only pauses for translation were for the Liechtenstein girl. Sometimes we had to slow down and clarify things, but we did well.
When I hear Italian or Romanian, I have to stop and listen, because initially, I’m not quite sure if it’s Spanish. That probably doesn’t happen to native speakers. On the other hand, did you know that background murmur of Dutch sounds really English? You’d never know it’s not English unless you tried to listen.
One of my grad school classmates was from Ecuador. His English was very good, but every so often, he’d stumble on a word. I’d ask him for the Spanish word, then chop the -io off of the end of it and hand it back to him.
I started learning English in 5th grade. In 7th grade we got French (I could have taken Latin optionally), and in 9th grade chose Latin as an additional language. My native language is German, and German helped me with English (though there are a lot of evil false friends), just like English helped me with French and both combined helped me with Latin. I didn’t learn Latin very long and very well, so especially English and French also opened quite a lot of Spanish and Italian to me, two languages I’ve never studied.
During the lockdown I started getting into Italian shows on Netflix, having watched several Spanish ones and after a while I started to recognise some of the words from my Spanish sorjourn. The curse words mainly.
I just reread the OP, and what surprises and interests me is that Italian has significant influences from Arabic. Spanish, of course, I can see, but Italian? Was it transmitted via Sicily?
Not as much as Spanish, but more than what I would have expected. Sicily, yes, but also through medieval trade. Remember Italians were the chief European traders with the Arabs sultanates.
Yeah, of course, I should have taken that into account, I’m reading a book about Marco Polo’s travels right now.
I was excited a few years ago when I was on vacation, I was finally going to visit a place in Spain and use my abysmal high school Spanish. Then we got to Barcelona (about a year before the riots) only to find they delighted in avoiding Spanish in favor of Catalan. Seemed about the same thing, except some (many?) different words, some more French-ish, and verb forms different too.
Not exactly, Portuñol is the blend of Portuguese and Spanish spoken in the Brazil-AnyoneElseInSouthAmerica border regions.
Perhaps the word you are looking for is “Porteño”? but I wouldn’t recomend calling someone from Montevideo “Porteño”…