Characters are Amberites; you can choose your parent, or leave it up to me; your choice. You may not be the offspring of unknown Amberites, but you can be children of Brand, Deirdre or Eric if you wish. You will have all been unknown in Amber before Patternfall, having appeared (in whatever fashion) in the years since then or during Gerard's Regency. Subjective age is up to you.
You will all have come to Amber at some stage and walked the Pattern there. You don't have to swear loyalty to Random in order to do this, but you must swear loyalty to have the perks of being a formally recognised Prince or Princess of Amber (as opposed to a mere Lord or Lady, which is what you will be addressed as otherwise). These include:
Note that no-one gets to bring in their own personal guards from outside Amber, or recruit guards for use within the castle who are loyal to them above the Crown.
None of you have had any real contact with Chaos, though you can have served in the forces of occupation there if you wish (or even led them for a bit, if you're good enough).
It would be nice to know where you come from and the like, plus the origins of any particular items of kit you carry about...
There is no such thing as a foreign language to an Amberite; as all shadow is a reflection of Amber, so all spoken language is a reflection of Thari. Thus Amberites can understand any shadow tongue, and shadowites can always understand an Amberite, all the time, even if the Amberite simultaneously speaks with shadowites who speak mutually incomprehensible tongues. Written language is harder for Amberites to understand, as it varies much more, but even so, with time and effort they can usually puzzle it out.