The gym I worked out at in Pattaya had several English speaking personal trainers (freelance) and most were busy training english speaking expats and nomads. Their rates seemed much higher than the Thai Personal Trainers. In the end, I hired a Thai trainer just to save money. He was excellent and very knowledgeable. Just have friendly conversations with people at the gym that speak a common language and offer your services. It's easy to identify who in the gym would benefit from your expertise.