Yes and no. Romanian had a Slavic layer in it, which you may count as a result of the significant interaction between Romance and Slavic populations around the

Yes and no. Romanian had a Slavic layer in it, which you may count as a result of the significant interaction between Romance and Slavic populations around the Carpathian region. French and more-so English, can be counted in the same way as the by-product of Romance and Germanic groups intermingling. I'm not aquatinted with the degree of Germanic and Slavic mixing, but I presume that there should be (or have been) plenty as well. Just remember that Poland become actually polish only after the deportation of Germans, at the end of WW2. The language groups haven't merged more because of limited movement and interaction of their populace, a political effect. The political reasons were also the ones that actually worked in the other direction - of isolating masses of people and encouraging independent development of their cultural identity. The conclusion is that linguistic convergence is not the only natural effect that you may expect with time. Take a large enough mono-linguistic space, and give it a couple of centuries. After that you'll start noticing enough differences to warrant acknowledgment of distinct dialects or even languages.