Lazarus/FPC (the IDE and compiler Lazarus is build with) are natively compatible with UTF-8 since version 1.6 of Lazarus and 3.0 of FPC. Maybe the next version of WinFF will be able to handle such filenames soon.
As for me, several years ago, I had to change my user name (François, which is very common also in France for a “Français”;) on Windows. Filenames or paths with “umlauts”, accents or cedillas are not compatible either, and WinFF sends command lines to FFmpeg, which does not simplify the problem. UTF8 implementation has been very troublesome on Lazarus for years, and I hope Matt will now be able to build an UTF-8 compatible WinFF with the new UTF-8 Lazarus…