I think a custom JS script is more appropriate here.
// pseudocode
discourse_api_on_load_i_forget_how_that_ones_written(function() {
if (user.locale == 'ar') {
var st = document.createElement('style');
st.innerHTML = "html {font-family: 'Noto Naskh Arabic', serif !important; }";
document.head.appendChild(st);
}
})