Had no problems in Germany with one account statement, proof of local address and a certification letter from the company incl business registration number.
You can’t teach in Thailand to to locals on the DTV. That requires a work visa and work permit. In order to be approved you need to show that you’rencurrently remote working and have certification of that.
You can submit all documents in your local language in the country you apply at. The embassy/consulate has staff capable to verify the documents. No need to provide any translations.
You don’t need 6 months account history in the U.S. that is only a requirement at some overseas embassies, mostly in Southeast Asia, to apply more scrutiny. A single month bank statement is sufficient for applying at U.S. based consulates.
that also depends on the embassy. Some don’t accept that. But if he plans pn moving to Thailand not sure how practical it is to use one single back account back home together with two (dependent) family members.