I used this app for a bit but refused to pay for it. It, like many apps, is designed to force you to pay for it eventually but I liked what I saw in it while I was using it. Just generally speaking though I refuse to spend money on apps because I hate doing things on my phone and don't like being around people who always have their faces stuck in their phones anyway.
But to the point of your post if that is the case, it is a wise one. I have quit many things in my life because they became too difficult. Also, them not having Thai as an option is likely a question of whether or not people would actually be interested in learning a relatively useless one-country language with special characters that are not used in any other language. Mandarin is a special character language also but considerably more people speak it. How many Thai people are there in total? Like 70 million? Not really enough potential customers there.
Even once you have paid for it they still have mechanisms built in to try to get you to spend more. I don't know how much money this company makes but I am sure it is a lot.
and yes, I can understand why Thai isn't offered. Of the under 1 million or so people that live here that are not Thai I would say only about a quarter of them express much interest in learning the language. That's not a big enough audience to bother with it.