Hello André,
I think you forget students (including one ones) that have to deal with such messages.
It depends which students. If they want to become programmers, they have to eventually master English. Many first-hand resources, and the most vibrant programming communities, are in English. (it's like a medieval theologian needing to know Latin)
Imagine you could start python with
python --lang="en" your_script.py
I would do the reverse and have Python start untranslated by default. Only by specifying an option (e.g. "python -L"), or perhaps an environment variable, would you get the translation suited to your system settings. Having Python translate its error messages by default, however, could be a disaster. It would break programs which parse those error messages. It could also make error reporting to a developer more complicated. And, as I mentioned, I've seen two many dreadful translations to like the idea of French error messages in my Python. (how do you like « déréférencement du pointeur type-punned brisera les strictes d'aliases » ?)
True ... I know I'd choose English as a default myself for Python (even though, like you I believe, French is my first language).
Mine as well indeed. And I'm not even Canadian :) Regards Antoine.