I’m a student of many languages, and notorious for being able to guess what someone’s native language is from how they speak/type in English.
In Spanish, I grew up hearing Mexican accents, and can recognize speakers from Puerto Rico and Spain. I know there are differences in the accents from other places, but I don’t know enough about them to tell you what they are. The Spanish-speaking Basque are also… interesting.
I’ve heard French French (in class), Canadian French (from friends) and Cajun French (also in class, albeit this one the anthropology course). Cajun French is actually easier for me to understand than the standard – it uses English-like cadence and the words are easier to spit apart.
My German class was all in Hochdeutsch. I find Bavarians completely incomprehensible, and Swiss and Austrian German-speakers have to switch to standard Hochdeutsch before I have any idea what they’re on about. Something we were watching in class featured a character with a Berliner accent, which I found both distinctive and easy to hear/imitate. It may or may not be relevant here that I think Dutch sounds like German spoken with a heavy French accent – I was working a dorm desk one summer when a load of media students came in from the Netherlands, and although I didn’t speak Dutch and couldn’t talk to them, I knew enough German to understand them reasonably well.
Standard Japanese (e.g., what the NHK newscasters use) is Tokyo-dialect, but accents from Osaka and Okinawa are featured pretty regularly in TV shows. Osaka-ben is an entity unto itself. Taking someone who learned Tokyo-ben and dumping them into Osaka without warning is akin to taking someone who learned standard American English and pitching them into the wilds of Alabama. Okinawan accents aren’t bad, but the local vocabulary in some areas involves a lot words borrowed from the Okinawan language spoken in the Ryuukyuus before standard Japanese invaded, which makes things difficult to follow.
My attempts to learn some Italian went kind of caterwampus until I realized that the friend who was trying to help me had spent part of his childhood in Rome, but all of the texts and CDs I had were geared toward the Florentine accent. They aren’t mutually incomprehensible or anything, but some medial vowels and word ends were different enough that I was never quite sure if I was hearing right.
I don’t speak Russian, but I know it must have some humdingers. I asked the Arabic instructor at my university exactly where she was from at one point, because her accent even in English was wildly different from the exchange student in my German class who came from somewhere in Siberia. The Arabic teacher turned out to be from Moscow, which judging from the vowels must be the Russian equivalent of Chicago.
Arabic, for that matter, has a number of recognizable accents. The same letter is “jiim” in Kuwait and “giim” in Egypt, and varies unevenly in between. Iraqis sound distinct enough that I recognize the sound even though my vocabulary is basically non-existent and I have no idea what they’re saying.
I’ve also heard from a couple of Norwegians that the regional accents there can get so bad they are reduced to speaking Swedish or even English to their fellow countrymen.