there is double-tax treaty between CZ and TH. And when I use personal money / savings, that was taxed years ago, I do not see how this would be taxable (again) in Thailand.
In Czechia you need documents that are translated and cross-signed by Ministry of Foreign Affairs and Ministry of Interior, and local Thai embassy could not even help with where they exactly sign papers like these... I gave up, locally. Documents itself are simple, but signatures / translations are not. Much easier to do in Thailand :)