The next guy with a different subset? :)

The subset is mostly defined by the jurisdiction you operate in, which usually defines a process to map names from one subset to another and is also in the business of keeping the log of said operation. The problem is not operating in a subset, but defining it wrong and not being aware there are multiple of those.

If different parts of your system operate in different jurisdictions (or interface which other systems that do), you have to pick multiple subsets and ask user to provide input for each of them.

You just can't put anything other than ASCII into either payment card or PNR and the rules of minimal length will differ for the two and you can't put ASCII into the government database which explicitly rejects all of ASCII letters.