Unless the DM tells you, flat-out, that the barbarian class mechanic is exclusively connected to the barbarian culture; which is entirely their prerogative, as world-builder.
I have another point of view to that bolded part: no it bloody well isn't!
As I mentioned when I first posted in this thread some 600 pages ago, DMs have control over everything in their world EXCEPT the PCs!
The PC's fluff is the prerogative of the
player,
not the DM, where 'prerogative' = 'final word'. The player (ideally) knows enough about the DMs world in order to make sensible decisions (so no cyberware fluff in a world without cyberware, no werewolf fluff in a world with no lycanthropy), and the player has to obey the game RULES set down by the books and modified by the DM (so if the DM says the barbarian CLASS is not available, then that's the way it is).
But fluff = backstory here. That's up to the player, within the realms of possibility. For barbarian culture
fluff, the only thing required is that there exists people with that culture in the DM's world. If the DM says that there are no barbarian
cultures in the world, then that's the way it is. But the lack of any barbarian
culture (which is a bit strange, bit this IS within the DM's purview) in no way prevents the use of the game mechanics of the barbarian CLASS, in and of itself. I could fluff my Bar 1 with (to take a popular example!) the urchin background. Certainly, not ALL urchins would use the barbarian class mechanic to represent them, but some would. Mine definitely does, and I know that because my fluff is up to me!
The only way a DM could prevent this is to ban the class AND the culture. But as long as the class is available, I can fluff it in any way that makes sense in the world. I could not fluff it as cyberware/lycanthropy without those things being in the world, but I have infinite idea space available to think of something that would apply.
Being raised on the mean streets is available, unless the DM's world has no mean streets on his world anywhere. Okay, no mean streets, no barbarian culture, but the class mechanics ARE available? Okay, I will steal my idea from the Deathstalker books, where Owen Deathstalker and the rest of his noble family have special organs which supply a special cocktail of adrenaline and other battle drugs at will, but they can only produce a limited amount per day. Maybe two lots at 1st level, three lots at 4th, you get the picture.
If the barbarian class is not part of the game, then the best representation of a barbarian warrior is that is uses the fighter class. If the barbarian class does exist, then the best representation of a barbarian warrior is that it uses the barbarian class.
If your character concept is of a barbarian warrior, and the barbarian class exists, and you try to use the fighter mechanics to represent that character, then you're being disingenuous. The fighter class mechanics are not the best representation of the barbarian warrior concept, if the barbarian class exists, and everyone should know that.
Rubbish! Not every single person in the 'culturally barbarian' tribe will have levels in the barbarian
class! Some will be rangers, some druids, some fighters, maybe even a paladin (vengeance?), while most will be NPCs without a specific class. The idea that a player creating a PC from this culture without any levels in the barbarian
class is somehow being dishonest is an unmerited slur.
Other unmerited slurs include: if you play a MC PC then you
must be:-
* powergaming,
* trying to spoil MY fun
* trying to change MY world
* forcing the DM to make ALL urchins have levels in the barbarian class
* etc. etc.
ad nauseum