8.3 file names on a NTFS volume are limited to standard ASCII characters, minus any reserved values.
You can alter this behavior, allowing extended characters, including diacritic characters, by using Regedt32 to navigate to:
On the Edit menu, Add Value name NtfsAllowExtendedCharacterIn8dot3Name as a REG_DWORD data type. The default, 0, does not permit extended characters. A data value of 1 allows extended characters in 8.3 short names on NTFS volumes.