@Cheryl
Bonjour, I will attempt to answer some of these questions in the hope it can benefit other ex-pats in France.
I live on the border of France and Spain. Most people speak French in my town, but many speak Spanish as well. English is not common.
The French language was more difficult to learn than I anticipated. Once you get past the archaic rules, or lack of, than you have to contend with a seemingly endless amount of conjugations. If that is not enough of a challenge, you have to learn how to pronounce correctly. Outside of the bigger cities where there may be more diversity, many of the French people are not used to hearing the language pronounced with different accents. Therefore when you are learning the language you will definitely butcher the pronunciations and the people will have difficulty understanding you. This can be very discouraging and damage your confidence to speak the language, especially in busy situations like supermarkets or boulangeries.
You just have to keep trying though and practice, practice, practice.
My tip for learning the language aside from the obvious ones of hiring a teacher or joining a course, is to listen to the language as much as possible. This means watching TV or listening to the radio in French. You really have to train your ear to understand the language. Also practice daily phrases and sounds that you have trouble with. The "r" sound and nasal sounds are not native to English and can be quite difficult to speak correctly.
I used flash cards to help me with useful phrases and words that I found to be used on a daily basis. This really helped me to at least have the basics of the language understood from a mimic standpoint. Even if I didn't always use the more "local" way of saying something.
The Dominant business language is French, but I am lucky enough to work with many people who are bilingual. For the first 2 years I didn't understand very much of what was being said in French, but over time with practice and persistence I can understand at least 70% of what is being said. It is not easy though to conduct important business meetings without at least a basic knowledge of the language.
I have found that proficiency is the absolute key to really understanding and even enjoying the local culture. Yes you can still enjoy many things, but you just miss out on so much if you do not know any of the language.
A few basic cultural nuances are quite known and common, but I will reiterate. First always starting a conversation with Bonjour or greeting. Never come in hot asking questions without doing this. If you walk into a doctors office, salon, shop, etc a simple "bonjour" announcement will grant you a welcome. Second, be conscious of your volume in quite places. The French just consider this as being respectful to others. Third, in a business setting always shake hands and look people in the eye before you conduct your business. If you're feeling brave a simple "ca va?" pleasantry will ease tension. Fourth, as compared to the USA you bag your own groceries at all markets. Be prepared with your own bag or ask to buy one. Fifth, it is generally frowned upon to eat or drink while walking. This is not a firm rule, but it will definitely show you are not a local. Of course water, or ice cream are the exceptions.
Last you can get by and survive without knowing the language but it will be very stressful and you will miss out on many things. If you are considering actually moving to France and staying for an extended time, it will be worthwhile to establish a basic foundation of the language before arriving. Most of the digital or textbook learning tools will teach you proper French, which nobody speaks, but that's ok. What matters is that you have a basic understanding of key things such as asking basic questions with regards to food, health, directions or buying something. Also spend some time learning the basic rules for future and past tense conjugations. Like I said, there's an endless amount of them, but as long as you learn how to conjugate I am, I will be, I was, Are you, Have you, When will you, Where are you, How do I, Can you and It was, this will help you enter the environment with a decent knowledge so you can at least communicate at a basic level.
Bon chance et bon courage!
