I can’t think of anything, it’s easier for Thai because it reads left to right, so at least you don’t have to mess around with text direction.
If you wanted to only load the Thai font on pages with Thai content, you can do it like this (change the ‘th’ for whatever code you declared for Thai in your config.)
<?php if($page->content('th')->exists()): ?>
<!-- Thai Font -->
<?php else: ?>
<!-- Other Font -->
<?php endif ?>
I am not sure happens with with the Thai language variables in the panel, I would think you would need to load in custom font there to. You can do this with a custom panel stylesheet. However, Kirby is multilingual so I think it will be ok without doing that.