You know nothing of being without friends.
Major language groups and dialects
All Tieflings speak infernal, due to the curse set upon them. Other languages that Tieflings know mostly depends on where they grew up and who raised them.
Shared customary codes and values
If society doesn't support one, any way to do it yourself becomes legit, be it hard work or crime.
Common Etiquette rules
Tieflings avoid conversations with other species like the other species try to avoid Tieflings.
Tieflings who meet usually stick around each other for a few days in order to exchange information. Getting information out of the network as a member of a different species is only possible if you pay enough. Information about other Tieflings has a very high markup.
Common Dress code
Since nothing is worse then being seen, Tieflings rarely go out, but when they do , they wear common clothes that hide their faces or at least covers their features.
Common Myths and Legends
The stories told only among Tieflings is that Asmodeus, overlord of the nine hells, once made a pact with a human. This pact was about power in the purest form, but there must always be a price. The price for power? A curse upon the humans loved child and every other child in the bloodline.A curse to be different. A curse to be violent. A curse to be shunned in almost every society.
The human who made the original pact with Asmodeus will never leave the minds of Tieflings, but the name has been forgotten. For many Tieflings this is for the best, because such a sinner should never have a name.
Due to the inherent difficulty to find a mate when most of society kicks a Tiefling out, the ideal relationship has become one between two Tieflings that lasts for a few years at least.