My last name is a technical hurdle. At least, for many people. In this high-tech, modern, globalized society, one would expect that a simple accented character shouldn't be an issue. After all, even a small child can write Stüvel correctly with a stick in the mud. So why is it such an issue in the world of computers?
To demonstrate the issue, here is a list of ways that I have seen my name appear:
The root cause? Often, software is only tested with what the developers & testers can easily type on their keyboard. As a result, they use the characters from the ASCII set, which happens to be the common dinominator between many character encodings. If you want to ensure your system correctly handles character data, test with non-ASCII characters. Go to Wikipedia, pick a random article in a language you don't know, and just copy-paste some text into your application. Your system should be set up to handle this. Preferably by simply using UTF-8, but as a worst-case it should at least let the user know that non-ASCII characters aren't supported.
And please, don't call those "illegal characters", it's not a property of the characters, but a limitation of your system. Those limitations are usually simple to lift, as long as testing with non-ASCII characters is part of the development process from the start.
Let's move into the modern world, and accept the view that there are more names than English ones.