The best situation for universities that are not in the US, UK, Australia, Canada, etc. is if the language of instruction on the transcript.
Otherwise, include a letter on official letterhead and also provide any additional independent verification of the language of instruction (i.e. the university's website or official policy showing all instruction is in English/French).
Letters in and of themselves are not the best proof, since letters, even on official letterhead, are not that hard to forge.