Two points:
There are now several commercial tests available (VAET, TEA, AEROSOLUTIONS, RMIT and others) and it's best to see which one is accepted by Turkey CAA before committing to any one. Be careful as some tests say the are "accredited" when this is not true. Check with your authority.
To become an Aviation English trainer requires no formal training as yet. In fact if you already have both an aviation background AND a language training bakground that puts you in a good position. If you wish to be able to "rate" the level of pilots using one particular commercial test you need to do a rater course provided by that test supplier. There is no official organisation of raters yet, and no matter what anyone tells you, you must remember that rating is still very unstandardised which means that pilots could do all the above tests and get different levels!