So the thing with Fantina is that you only know that she's supposed to be foreign, because in the French version she speaks English. Literally only 2 people are known to be foreign to the Pokemon world and have ties with our own world:
- Fantina, who speaks with a foreign language
- Lt. Surge, who is credited as being American
However, Fantina's nationality is never made clear, unlike Lt. Surge. Fantina was also introduced relatively late in the Pokemon franchise, in Gen IV, whereas Surge was introduced in Gen I.
This makes me believe that the creators of Pokemon didn't know exactly how to deal with Surge being American, despite making him so. If they introduced many more different nationalities, they'd have answer how the Pokemon world links to our own, and how Surge managed to get from ours to that world, or if being "American" is the same as being from another region in the Pokemon world, and if so, why haven't we seen that region? Or if we have, why is no one else from that region dubbed American like Surge is?
Henceforth, even though Fantina is foreign to Sinnoh, she is never explicitly French, meaning that the creators can avoid the question. "how did Fantina/ Surge travel from our world to theirs".
I also believe that there might have been certain concerns with how close the Pokemon story line (albeit non-existent haha) would be far too close to the Digimon story line if the creators made it so Surge & Fantina came from our world. Remember there used to be so many news pieces and rumours about Digimon copying Pokemon? That's why I feel like Pokemon never introduced another character background like Surge, because then Digimon could reverse the claim, so this is just my speculation.
As for how the worlds link together, here is a really nice map someone made that fits all the regions together:
Tumblr source (I'm not sure the OP is the person who created it though)
However, this is probably rather inaccurate, considering that Unova is always quoted as being significantly isolate from the first 4 regions. In the map above, Unova is closer to Kanto than Sinnoh is. So, there are a lot of fan interpretations as to what the Pokemon world looks like with all known regions (from main line and side games), but there isn't an official map that shows all regions in relation to each other (that I can find, anyway).
I also know that a lot of fans deem Kanto, Johto, Hoenn and Sinnoh to be in the same "country", because:
1. Kanto & Johto are very close together (note: GSC and HGSS)
2. There's an ace trainer somewhere in Hoenn who's trying to get to Johto from Kanto (or the other way around) and instead ends up on the edge of the Hoenn map
3. Sinnoh has many Kanto/ Johto/ Hoenn Pokemon as well as its own native Pokemon, which Unova does not have at the beginning of BW (you could probably argue Kalos is near the first 4 regions too on this basis).
Basically, it's open to interpretation, because the creators left Pokeverse geography rather vague, intentionally or unintentionally.