Seems like the text file was saved with the incorrect encoding. If you're using default Notepad, make sure that when you save the file you select ANSI. Since your computer's non-unicode language was already set to Japanese, this means that any Japanese characters in the file will force the file to get saved as Shift JIS.
If you're using other text editors (like Notepad++ or VS Code) you can directly set the encoding to Shift JIS.