i dont doubt the safety aspect of 1 language atc. its the best. but i also agree on multi language atc and here is why.
ive noticed that on my little island, there are a lot of venezuelan GA traffic, these guys are cowboys straight from a eastwood movie.
if you dare to speak english to some of them, they will answer you in english, but in reality, they did not understand a bit of it, and won't comply to what they read back.
dont ask for examples, just use your imagination.
now, the atc notices this, and will instruct immediately in spanish, and then they do comply.
so in "many" cases, i've noticed that using more then 1 language improves safety.
i just wished that all pilots would care enough for this aspect and improve their language skills. meaning learn english.
and then for the english speaking ones, no harm in learning and understanding basics of other languages spoken in the country ure working in. not all obviously.
i speak 6 languages 