Is there a language barrier for placing orders or speaking to a Thai Broker in English?

No.  Brokers here are called "Marketing Officers". You should be assigned an OK to good English speaking person, and most have a University Business Degree. Thai Broker Research Reports, Monthly Broker Statements, and Annual Reports, are published in fairly clear English but this varies much from broker to broker and some are only in Thai language.  Any instructions to buy/sell shares can be given via fax, phone, or in some cases e-mail or even e services like What'sApp.  Most all brokers have on-line trading, which I advocate.  You best test their system to see if you like & understand it.  As everywhere, its always most important to enter every buy/sell order very carefully & clearly so to avoid any mistakes. (January 2 2022).