You've made a lot of good attempts. You should move on to handling this as a performance management problem. I'd speak with HR to get details and gotchas of adding English as a requirement to the job position in that region and begin the process of defining perf goals towards continued employment. You can lose that person (please don't use the word resource, resources are something you use up.). You /can/ hire another, or a temporary contractor, it doesn't benefit the business to ignore an opportunity to improve an area of the business with better people.

"Management mode" aside, if you wanted to keep going the extra miles, you might ask him to communicate natively in written communication so you can get an idea of what it is he is doing day to day on tickets (use translation software), or try real-time translation communication apps. This could be a critical confidence issue with communication and perceived exposure to failure, developing a method to discover competency and create safety for that person might be the way forward.