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 a OK to good English speaking person, and often have a University Business Degree. Their Research Reports, Monthly Broker Statements, and Annual Reports, are published in fairly clear English but this varies much from broker to broker. 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.  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.