You can get 95% with no effort at all, 99% with a bit of thought, 99.9% with some effort, 99.99% with a smart design, some research, and a lot of effort, and anything further requires making it your full-time job indefinitely.
It gets thornier when it comes to personal things like names and gender. If you tell a group of people that their identity isn't supported by your system, some people will accept that and work around it, some people will be mad at you, and you'll just have to accept that the latter have a good point. You have to stop developing it at some point, so all you can do is try to cover as many people as possible.
That said, it is incredibly revealing of your personal biases if you design a system that breaks on things like names with "é", or "ñ", or punctuation; or if you implement gender as a boolean that also determines the user's pronouns.