> they don't like to hear even the slightest imperfection in pronunciation or usage, seeing it as some sort of defilement of their language.
As a native French speaker, this strikes me as odd, since it is typically a foreign thing to consider French as the epitome of spoken languages, that makes most French people I know scratch their heads uncomfortably.
I interact with foreigners everyday, most of the time in English. Most of them are PhD students that spend at least three years in France, and usually acquire some French along the way, to varying degrees of proficiency (from greetings to highly technical/rhetorical conversations)
I know some of them that handle it well enough for basic conversations, and I will usually go along with them until it becomes difficult for either party to understand the other.
Actually, I might be a bit too eager to switch back to English as I have a stereotypical view of their possible interactions with other French people, who might not "excel" at English. Usually, I will not correct them on a word unless it was the root cause for a misunderstanding, or they asked me to tell them when they do a mistake. I am also happy to discuss grammar and etymology topics, as I am usually curious about other languages as well. I do not want to keep them from learning French, nor force them to speak it, so I'll play along what I guessed their intentions were, based on our previous interactions.
On the other hand, we are taught French at school with very rigid rules, which are tweaked from time to time (by the "Académie Française"). This helps with uniformization, but could be argued either way. And finally, it might be a bit elitist of me, but a lot of French people struggle with proper writing (French is much harder to write than it is to read or speak), which I think looks very unprofessional and neglected. Quite a lot of people will frown on this (at least those that make fewer mistakes), though foreigners usually get a free pass (from myself at least, not quite sure about others).
I hope that clears things up a bit. I find it a very interesting topic, as are the issues of social integration and forming small cliques.