Put the steroides aside and don't make it so complicted.
1) English based RT for all professionals (it's a good exercise for every non native English speaker). Mixing RT languages will continue to cause problems
2) native english ATCO to non native guest: treat them as such. From the first contact you're able to judge what proficiency level you are dealing with. Leave your John Wayne and do some of your highschool english.
3) native english speaking pilots visiting: behave like a guest - same remedy.
Story time........This morning in a southern spanish fish market I visited, the probably poorly educated girl rattled something after handing over the order. Within a split second of seeing my puzzled face she repeated 'dosss EUROOO cciinnncuuentaaa'. Exchange was kept brief and a quick smile cleared me for a swift departure, the next client lining-up right behind me. She could have handled LAX/LHR/ORD any time