try to explain the term "Bildungsurlaub": Germany has a federal system. Some of the federal states (Bundesländer) have the law "Bildungsurlaub". These meens the employees can asked for 10 days off, to learn something (a language, something technically etc). The employer has to pay the normal salary and he does not have the right to cut the holiday of the employees. In Germany normally the people take the 10 days to take part at a intensive language course, normally in English, French or Spainish. These are the common EU languages.
So I am very astonished why somebody like Thomas wanted to send customers to your school in Bangkok? First 10 days is not enough for a travel and a language course in Thai. Second, nobody in Germany is interessed in learning Thai, or very very few companies or state offices need people who can speak or understand Thai. Perhaps in the tourism business, but here normally they have Thai employees who can speak German/or English. So I do not understand the motivation of Thomas.
He can not make money with your certificates, because to get an approval for your language school, you as the owner has to apply. There is a special formular. With this formular you as the owner of the school can asked the German Governement Office to get a special permission to offer Thai language courses for German persons, who want to take the "Bildungsurlaub". But in my view this is not big business. Because, like I mentioned before, there are very very very few German people who need to learn Thai.
I would like to write a letter to Thomas to asked him why he wanted to offer your school, why he thinks with "Bildungsurlaub" he and you could make business. I can not see big business.
Sorry for these not so nice informaiton
best wishes