Given you low post count, you may not be able to post screenshots yet...
One way you could do this is the (very) old fashioned way...
Download "
Terminal Emulator" from the Play Store.
Open it, and type the following commands:
this will get you to the downloads directory
For clarity, that's a lowercase "LS"; this will list the contents of the directory, you should see the file in question listed (it may not show the Japanese character)
Now, what you want to do is run an "rm" command to delete the file, since you can't type the Japanese character, hopefully you can use a wildcard instead. So, for example, if the name of the file is "abcd[foreign character]efgh.mp3". Do you have any other files that start with "abcd"? If not then type the command:
That is lowercase "RM" this will delete ANY file in that directory that starts with "abcd"
Just replace "abcd" with the first few unique characters of the file in question, up to the Japanese character
You could also start with the wildcard, and use:
this will delete ANY file that ends with "efgh.mp3"
Hopefully that should do it...if that doesn't work, you could try to use the "-f" switch after the rm command to "force" it, but that shouldn't be necessary. I'm not sure what I'd do if this didn't work.