It is important when we think about our ancestors that we don't make too many assumptions based on our modern situation. Many Americans live in a very monolingual world. We speak and write almost exclusively in English and every thing that documents our time here is written in English.
However, this is a peculiarly modern situation. Before the 19th and 20th centuries, most people lived in large multilingual empires. In the British Isles today, Celtic languages like Gaelic, Cornish and Welsh have been relegated to often shrinking communities at the peripheries, but if you were to go back to Shakespeare's time you would find large areas where languages other than English dominated. For example, as late as the 1890's Scottish Gaelic was the majority language throughout the Scottish highlands.1
Be prepared for a degree of complexity that may be quite strange to us today. Many of our ancestors would have encountered many different languages in their lives. Records may have been produced in several different languages. It is very common for church and synagogue records in the same place to be in one language and government records to be in another. Throughout the world, Catholic church records were commonly kept in Latin. For my Schoendorf line in Saarland, I have church records in Latin and civil records in German and French. A single area may have changed hands, sometimes several times, and each time with a change in the language of administration. If your ancestors lived near a border, don't be surprised to find that the languages of the records changes from time to time.
To make things more complicated, your ancestors may not have even known the languages that these records where in. In some cases, they may have spoken a dialect very different from the standard form used in writing or they may have spoken a completely unrelated language. In other cases, your ancestors may have been comfortable in more than one language, switching from one to the other depending on the situation or need. The situation may even have changed across generations.
As always, be careful what you assume. Just because your ancestors were from France doesn't mean they spoke French. They may have spoken a language like Basque, Breton, German or Flemish. Conversely, just because they are from a region where one of these languages was spoken doesn't mean they didn't speak French. You have to weigh the available evidence carefully.
1. Kenneth MacKinnon. "Scottish Gaelic Today: Social History and Contemporary Status," in The Celtic Languages, ed. Martin J. Ball (London and New York: Routledge, 1993), 491-535.