So, you want to distribute your program across multiple platforms but don't care whether it works there or not? Why are you asking at all then?

Note that about every OS (respectively filesytem) nowadays allows "accented characters", and many filesystems have allowed whitespace in filenames and directories a long time.

It seems to me that you don't have much experience with users, as users like to put whitespace in their filenames and also like to write names as they are properly written instead of asciifying them. It depends on you whether you want to educate your users about how to properly name their files and directories or whether you want your program to just work with what they throw at it.