[BrandoElFollito]

In France it is virtually impossible to function of you do not speak good French.

In a company you will miss all the important things that are usually conveyed in casual French (this includes subtleties that change the meaning of a sentence).

Yes, people will speak English with you, usually fluently (and with plenty of errors), but this is not the language of choice for the off the record conversations.

Outside of work you will get by in supermarkets, but for more complicated shops your will have problems.

For the administration it is clear: without being fluent in French there is no way to do anything. Even if you are a native speaker you understand just half of it and hope for the best.

People in the street speak English [citation needed] but they will get tired after some time and then the English becomes unbearable.

Source: native French married to a foreigner who struggles to understand everything (not to mention the subtleties) after 15 years of active use of the language.