Before you go, go on line and find a site that gives you a dozen conventional and politeness phrases. You MUST great people on first encounter -- walking into a shop, getting croissants at the bakery, asking for directions -- ALWAYS start with bon jour (or bon soir). and you need to be able to say thank you, please, excuse me and ask basic directions. These are easy to know and there are many sites on line that list them and also will play you a recording to hear how they are pronounced.
Most people at tourist sites will be able to cope in English although again -- you must use the politeness phrases first. In less central spots that is less so. We usually stay on the fringes of Paris and these days more and more people have a little English but even so it is not unusual to encounter those who don't.
The phone translator is not useful in the middle of a conversation usually but you can prepare a question or request or use it to figure out menu items. FWIW. you are usually better off just googling the dish in question than using the translator. T he translator won't be able to translate a dish with a name like Gratin a la Maxim -- or whatever. But google will often show you the recipe and a picture.