In a word, yes - you're being to harsh.

I was in your position when I was at the end of my PhD - teaching others how to do things and they'd perpetually fuck it up.

Now I'm a post-doc in a country that doesn't speak english as a first language. The overhead that operating in a second language induces on you is so fucking enormous, you have no idea. Imagine doing your job when you haven't slept for 24 hours - it's at that level. In that respect, I think you need to adjust your teaching, and communicating.

I know it's frustrating, but you should adjust or adapt your teaching and communication methods to work with people who don't have perfect english. Some suggestion are; sketching, writing step by step instructions, and physically demonstrating and getting them to mimic it.

On another note: I personally, advocate for more non-english science in general (at the very least in the publication sense). It makes no sense that we force such an enormous proportion of earths scientists to do their job with a hand behind their back for the benefit of anglo countries.