I think the misunderstanding here is in thinking ChatGPT has “languages”. It doesn’t choose a language. It is always drawing from everything it knows. The ‘configuration’ hence is the same for all languages, it’s just basically an invisible prompt telling it, in plain text, how to communicate.

When you change/add your personalized “Custom Instructions”, this is basically the same thing.

I would assume that this invisible context is in English, no matter what. It should make no difference.