Read this and answer the question. If the question is unanswerable, say "unanswerable".

From the annexation of Alsace by France in the 17th century and the language policy of the French Revolution up to 1870, knowledge of French in Alsace increased considerably. With the education reforms of the 19th century, the middle classes began to speak and write French well. The French language never really managed, however, to win over the masses, the vast majority of whom continued to speak their German dialects and write in German (which we would now call "standard German").[citation needed]

When did French become the most common language?
unanswerable