Thanks for clarifying, I did not know that. Now I have some words to search for. 
I do see that there’s an open issue about IDN support and non-ASCII characters. But it hasn’t gained traction yet (so also not confirmed yet).
I learned from this topic that there’s a removePathAccents setting for your config.toml file that might help. Although the use case discussed in that topic isn’t exactly the same as the one you have.
I couldn’t find documentation for that variable, but a Google search did turn up this article. Based on the description there I’m not sure if it helps in this case (but always worth a try I think).
Other than that I don’t know how to use non-ASCII characters in Hugo URLs.