It's not a common expression in modern English.
"All that" means "all the words that I just spoke". And "French" is used jokingly as "language that isn't understood". So he means:
All this talking about "conceited brain and too little stomach" is hard to understand. But to make it simple for you, it all means that you should take this liver pill.
A "liver pill" is actually a laxative, and were often sold as "cure-alls" by charlatans. The whole monologue is "patter": fast talk to get someone to buy a product.
The first part of the patter is a modification of a line from the Bible "faith, hope, charity, these three; but the greatest of these is charity"
Almost any foreign language could be substituted in place of "French" but it would be natural for an English speaker to refer to a language spoken close to England: French or Dutch would be natural, or even "double dutch". "Latin" or particularly "Greek" is also common, from when these were commonly taught to high achieving pupils at school.