Apologies for the problem with registering a domain with Spanish diacritics. Actually, you can register a domain name using diacritical marks. You need to basically convert the domain name using PunyCode. Here's a site that does it - Punycoder.com.
So, for example:arñelstestdomain.com
Would be converted to: xn--arelstestdomain-zqb.com
You can then go to a domain registrar and register that domain name. In some cases, some domain registrar will automatically convert it for you. I believe that Namecheap.com does this. Even though the domain is registered as xn--arelstestdomain-zqb.com, it will still be recognized when you type arñelstestdomain.com.
You can look up more information on the topic: International Domain Names or IDN. I hope that helps to explain it.!
If you have any further questions or comments, please let us know.