SEO Friendly for Other Language

[NextGen]

I have used prestashop v.1.2. It works very well
But I would like to ask how I gonna input like Thai character for SEO-Friendly

Because everytime I type in down it always automatic delete non-english character

First I try to edit AdminProducts files in Admin Tabs

and

Validate.php in classes but it show up the errors. So I stop it, because I don't want to mess up my shop

Please anyone know how to do this, Thankyou

PS. Sorry for my bad english ( It's my second language )