I think asking them to work on the letter together would be a good idea. There were two vets at one of the places I interned, and what they did was have one (the more senior vet) actually do the evaluation, while the other vets and the techs gave her comments to help with the letter and to fill in the blanks of the evaluation where it was asking about things that she hasn't personally observed.
There's a sample evaluation
here, so as you can see you won't really have to worry about telling them how to do the evaluation and letter, since there are instructions.
As for the language aspect, I'm not sure. If you ask the vet who is better at english to actually do the evaluation and write the letter, with the other vet proving extra comments and thoughts, that might solve that issue. Though it depends on how good at english you think he is.