Hueyracer - the FAA statement does not satisfy ICAO requirements because it does not state a level. Even then, it is assumed at level 4, which expires after 3 years. Rapidly approaching for most people.
Some countries will only accept a (re)validation that is done at a "flight test", not a "review" or "check" - so an FAA certificate will never be acceptable because it is never stated when, or how, the evaluation was made.
The FAA guidance notes specify that an AME must evaluate a pilot's English proficiency at each medical. But nothing is actually done unless the subject is deemed to be inadequate - so no proof.
The whole thing is the usual dog's breakfast with the biggest player ignoring the rules.
The UK CAA store your level details but don't print them on the licence. It is a pilot's responsibility to know their level. The only way to "prove" it is to pay the CAA to divulge your level to your choice of AA for a validation issue.