This is related with the subtitle file’s encoding.
When the app opens a subtitle file, at first, it tries UTF-8 encoding.
If it fails,the app tries again with the encoding related with your phone’s language setting.
If your phone’s language is set at Spanish, it tries Spanish encoding.
Now if your subtitle file’s encoding is not neither UTF-8 nor Spanish encoding, then characters can be broken.
Since UTF-8 encoding works always, you may change your file’s encoding to UTF-8, then try the app again.
To change your file’s encoding to UTF-8,
Open the file from notepad of your PC.
Check all characters are okay. (if you see broken characters there, it can’t help…)
Then execute File(F) -> Save As(A) menu.
Then a window you can set a new name will pop.
At the bottom(left of save button) of the window, there is a encoding button.
Open it and select UTF-8,
Save a new file, and upload it to the app again.
Now you would see all subtitle characters without any problem.