1 connect in ssh to your raps
2 go in your skin folder (mine is /usr/share/kodi/addons/skin.osmc)
3 edit file 16x9/Font.xml
4 copy the default fontset or Arial fontset, change all ref to Arial.ttf by your font (I suggest arialuni.ttf which is exactly like Arial but supports Asian characters, at least thai). and append at the end of the file (before closing anchor font).
6 change in osmc settings-> appearance, font arialumi is available and it works fine !
good luck