When you want to emphersize the plural you can use 们, otherwise it is no need.
In your sentences法国人说法语.It's no need to emphasize plural.
我的朋友说法语。（in this sentence, it's not clear that I talk about one friend or many friends.)
我的朋友们说法语。(This sentence shows I have more than one friend speaking French.)
我的朋友都说法语。(This sentence has the same effect with the second.)