Of course I h,ve heard Asians speaking English with complete command over the language.I dont understand why you are being so prejudice even native speakers may have wrong pronunciation,weak vocabulary and may be unaware of grammar rules and it can be with a speaker of any language.
by the way who wants to be a language teacher without having an adequate knowledge in particular field.Other subjects if taught in English require a certain level of understanding but for sure language teacher might not be suitable for teaching Maths,General science,History or Geography.A person having degree in social sciences can play a better role in these fields with special courses in International Primary Education.