Arabic or Persian subtitles not displayed correctly when using SubStation Alpha standard files

I am using .ass files as I need to display multiple languages at once (4 languages, one in each corner!). Everything works fine on PC but I tried yesterday evening at home on my Fire Cube as we had guests from various countries and, surprise, Arabic and Persian subtitles are displayed as squares ... Might be a font issue but I am using the Arial default ... And if I use an .srt file with only one language, everything is displayed correctly. I have spent the day trying to solve the issue but to no avail. Set the encoding to match the .ass file encoding (UTF-8), converted the video file to Matroska in order to embed the subtitles, I even tried to specify "Helvetica Neue Regular" as the font as it is the system font used by Amazon, nothing works...