We build Slicer with UTF8 application code page on Windows, and UTF8 encoding is used on Linux and macOS by default already. So everything, including VTK, CTK, ITK works well with non-ASCII characters.
If your Linux or macOS system is set to a non-UTF8 locale then you need to restrict yourself to ASCII characters or switch the locale in the environment where you launch Slicer.
There is still an issue in Windows passing command-line arguments from a non-UTF8 terminal to a UTF8 application, but this should be resolved as Windows is gradually moving towards UTF8 everywhere.
What is the output of locale command on your system (in a terminal)?
What is the output of this command in Slicer’s Python console?
Update: I’ve found that there is an intentional stripping of special characters in the CTK DICOM exporter:
While the intent is good (you don’t want to create a folder that you may not be able to write to a DVD or USB stick later), but may lead to other complications. I’ve pushed a few fixes, you can try if it works better in the Slicer Preview Release that you download tomorrow or later.