There are 127 characters in ASCII and tens of thousands of characters in the real world. It is probably an interesting debate, trying to come up with the most efficient way of encoding non-ASCII characters without screwing everything up.
Don't waste your time. Use UTF-8 and Unicode.
"But what about UTF-16?" No.
"But what about--" NO.
ASCII is included in UTF-8 Unicode. So is everything else. Everyone understands it, everything's assuming it, and all the other encodings and charsets are more obscure and therefore harder to deal with.
Everyone (except PHP) has UTF-8 Unicode built in to whatever programming language they're using.
Unless you're writing for devices with memory measured in bytes and a network connection measured in baud then you have time and space to use the bloating of UTF-8 Unicode. So suck it up, be inefficient, and accept the VHS of UTF-8 over the Betamax of whatever you're looking all cow-eyed at today.
And, in case you were wondering, ASCII is never the right answer.