Recently I sorta failed on an attempt at hypnoing somebody. I got this idea of trying to do it in English next time, since that's a second language for both of us and I'm hoping this would encourage more attention or focus on my words.

Just a thought by now, but I think some input would be great. Would there be any precedents or studies for this sort of thing? Could it work or am I delusional?

...or just plain comments and suggestions?

I actually don't see much good coming out of that. It would certainly make them pay closer attention to your words, but if the language is their second language they may need to put too much attention on consciously interpreting the language to actually go under. I think I remember reading a post somewhere in which the person was hypnotizing someone who turned out to speak English as a second language and they started speaking their native language while under. So, the subconscious might not readily understand what it is being told in another language.

Additionally, depending on how fluent the two of you are, you might run into communication problems due to understanding some words in different ways.