UTF-8 format works, thanks!. But for some reason, German language WTT file does not show when trying to upload in wordpress (‘add media’) to reference in fv player shortcode. I speculate that WP rejects characters in the wtt file and prevents file from being uploaded as media.
the extension should be .vtt. We whitelist that extension in FV Player, otherwise WordPress wouldn’t let you upload that file. So if you named it .wtt by accident, it might fail to upload for that reason.