Do you have a professional background that utilizes this sort of knowledge or is it a hobby?
A bit of both. I’m a historian by background and I did several courses on Mediterranean history and my thesis had to deal with language questions and nationalism. In addition, I just like Mediterranean history and languages in general, and my hobbies (late Roman historical re-enactment and DM’ing Cthulhu and, in the past, Ars Magica, both involving Mediterranean history, European languages and dialects) reinforced this.
Antiva sounds like a country with "south European accents since they sound all latino" so it must be all the same.
All the countries and cultures in DA, with the exception of Ferelden (‘America with swords’) are effectively ‘exotic’ from the point of view of the creators and the main audience. The DA games are made by an overwhelmingly white Anglophone North American team (there’s an occasional Brit here and there, I believe) for a primarily white, Anglophone North American customer base. So yes, there’s probably something of a North American ‘all you Mexicans / Colombians / Spaniards / Italians / Greeks / Arabs / Indians etc. all look and and sound the same’ effect going on here.
Now to be honest, even northern Europeans (particularly the ones that aren’t particularly well-educated or never go beyond Marbella beach) sometimes show a bit of this attitude.
At least Bio tries to bring in a little diversity; let’s be glad they don’t have the cultural insensitivity issues we see here:
https://www.youtube....h?v=SeoX8MZd81E
(Though I have to admit I had to chuckle, particularly at the Amsterdam scene)
Do we have any reason to believe Antiva's the size of Corsica?
Bio has been very fuzzy on world size; there’s probably a reason why the markings along the edges of the Thedas map don’t come with any indicator of the scale. The only indicators we have are travel distances in DA:O and one in WoT. The first allows calculating that Ferelden is about the size of Puerto Rico, and Antiva is somewhat comparable (and thus about the same size as Corsica). The WoT reference is too non-specific to allow reliable calculations, but is in principle compatible with the DA:O-based calculation. See here (http://dragonage.wik...ig_is_Ferelden? See also this thread, starting about here: http://forum.bioware...4#entry16806070 ).
Personally, I don’t think Bio intends Ferelden to be this size, but it does mean that Thedas’ size ‘hangs in the air’, and the only info we have indicates a small size.
I also don't think you need much of an explanation to explain a very large degree of linguistic differentiation within a single nation, either, especially if it's an analogue for a European nation. Spain's got Castilian, Catalan, Galician, and Basque (which isn't related to it). France has several languages, including Occitan, which is more closely related to Catalan. Same with Italy.
You have to make a distinction between the existence of multiple languages side by side, and their actual mutual relationship and origins. Spanish and Italian accents suggests the existence of languages similar to Castilian and (Tuscan) Italian, which in our world have a history of over 2,000 years of linguistic development in regions over 1,000 kilometers apart and divided by sea and land.
Castilian and Italian had no meaningful direct language contact before the 16th century. Prior to that time, Northern French and Italian (specifically the northern dialects, including Tuscan) had more language contact with each other – through trade, conquest and settlement - than Castilian and Italian.
The really, really tricky thing here is not the existence of two languages in a single country, but the (implied) existence of two closely related but very distinctive ones within what looks like a rather modest-sized country. That requires time and / or distance, and a prolonged absence, or severe limitation, of extensive language contact.
Regardless of its exact dimensions*, Antiva seems to be relatively modest in size and has a clear centre. The local emergence of two very, very distinctive languages, resulting in two very distinctive accents in English / Common seems pretty unlikely under these circumstances.
Again, unless you whip out the old ‘migration from elsewhere’ excuse.
Look at this:
Whoa. I just fully blew my own mind. Differing Antivan accents are plausible as hell.
I advise blowing your nose, not your mind, much healthier
.
A random dude or dudette superimposing part of the Thedas map on a map of Europe doesn't mean a thing.
Anyway, even a modern language map does not give a good impression of how things ‘are on the ground’, and particularly not how they were historically or how they developed. Galician, Portuguese, Castilian and Catalan are far more similar than Castilian and Italian. I certainly can’t differentiate between the English accent of a Catalan and that of a Castilian with similar levels of fluency, though perhaps if I really, really pay attention I just about might**.
If, ultimately, you want to say ‘who cares, it's just a game and anything goes’, then I would agree. DA is not about linguistic consistency, it’s about killing things, looting their stuff, silly banter and occasional virtual sex. And nugs (let's not forget the nugs)... 
* Antiva is described as a city-state if I remember correctly – it sounds more like Venice, Milan or Florence than the 13th century lands of the Crown of Aragon or 12th-13th century Sicily.
** Next time I see my Catalan friends, I’ll ask how to recognise the differences. I will probably have to be diplomatic about it