Currently Flarum URL "slugs" are ASCII-only, generated from the title through a process called transliteration.
There have been comments in the past about improving transliteration and we will certainly make it as easy as possible for extension to extend the transliteration. Currently transliteration can be improved by installing the PHP intl extension, or using the Laravel macro to override the Str::slug method to define a custom slugger.
However using UTF-8 characters directly in the URL is something different which I don't think we will add as a default option. Extensions can already provide alternate URL slug strategies, so it's easy for an extension to do this as well. The challenges here might be URL encoding/decoding and browser support.
For forums in languages that can't be transliterated to ASCII, the best built-in option is probably to choose "ID" as the slug driver in the settings. We are leaving the development of more advanced options to community extensions.
