I think that reading in English would be easier for you, especially for that sort of things.

Reading it in French would help you to get some scientific vocabulary in this domain (and God knows what a pain analytical mechanics is), but it should be easier for you to read the book in English, in order to grasp the concepts quickly. There should not be any significant loss between the translations, so you will not miss any of the concepts detailed inside the book.

Learning French before reading french material helps you to understand the meaning of the sentences, especially if you focus on the vocabulary. Some words are quite complex when you read scientific material.

Reading the original makes sense for literature (and journalism), but not for reading works of science. I wouldn’t want to read Marcel Pagnol in English (bought a translation for a girlfriend once – disaster), but I think you’ll be alright with Lagrange!