The French translation of a "CHATELAINE"(n) is a Lady's chain for suspending a watch or a piece of jewelry ect. A "CHATELAINE" watch can be a Ladies pendant watch, or a pocket watch.
The CHATELAINE is just the belt it hangs from.
Here is one, that I donated to the NAWCC and is now on permanent display!

The watch is a small ladies Jaeger-LeCoultre, fitted in a "Niello" case to fit on a "CHATELAINE"