arial.ttf does not support Thai. You are probably looking for arial unicode.
Thank you, you are right.
After copying arialuni.ttf to /storage/.kodi/media/Fonts and renaming it to arial.ttf and rebooting the RPi4, Thai subtitles are working now.
Arial Unicode MS font family - Typography
Typographic info for the Arial Unicode MS font family
learn.microsoft.com
But there are still placeholders for the LibreElec UI (Estuary skin), i. e. pathnames or filenames.
How can i prevent those placeholders?
Thank you!