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 many 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 order on-line, phone, or in some cases e-mail or even e services like What'sApp.  All brokers have an on-line trading platform, which I advocate.  But best test their system to see if you like & understand it. "Streaming" is the SET standard default system.  As everywhere, its always most important to enter every buy/sell order very carefully & clearly so to avoid any mistakes. (July 2 2023).  Beware of broker equity research -as it can be less than hoped for.