Can't say if this is different to the Windows version, nor if it was mentioned before. To me it is a typical "problem" that appeared when porting Windows features into the Mac world (I still don't believe that MediaCenter was natively written for Mac):
As I understand the video tagging feature, everything is written in xml files, which really is a good idea. The whole tagging and film information system brings up different windows with fields wher one can add something. For example the "Get Movie & TV info..." part. Opening this window puts the file name into the upper field. This helps to find information. MC here does not understand any other character than "native" English ones. Foreign characters like those with dots and strokes above them are not recognised, although they are part of the file name and thus are part of a commen name system and nothing unusual.
Also: It is not possible to write typical French letters into the tagging fields that become the xml file. Letters like à, é, ê, etc are just ignored, but can be added manually to the xml file. Yet they will be ignored by Media Center if they are in the xml file.
Maybe this has been repaired with a newer MC version.