'converting' US luminairesmay be unsafe, as they may not be designed to carry 230 volts.
But to convert UK luminaires ( as well as grounded american ones) what you need is a proper french or swiss plug. Does your place actually use french plugs? the swiss don't use french plugs, they use Swiss plugs.
French plug and socket:
Swiss plug and socket:
Are you sure you don't mean 'europlug' by french plug?
Which is an ungrounded plug designed to work in most EU sockets.
Sorry if i was being patronising by asking you, but most people don't know of the different earthed plugs around Europe, they just see europlugs. Certainly it's not uncommon to find french sockets and plugs in switzerland, but swiss plugs and sockets I would imagine are far more common.
What you need to do is buy the correct grounded (as us europeans say 'earthed') plug for your sockets.
If it is the french one, you'll notice there's a hole in the plug, and a pin in the socket, you want to connect the earth to the hole.
If your sockets are unearthed, you could really do with a rewire.
Some notes as regards polarity:
These three pin plugs are polarized, unlike europlugs. Now, if your american lights came with a polarized plug, you should use these and observe polarity. However, you should check that all parts of your american lights to make sure they are safe for 230v. Are the cable, lamp connector, switches, etc. all rated at european voltage or not?
If you're not sure, don't use it.
A small piece of lexical nazism:
A lamp refers to the bulb, tube, or whatever.
I assume what you are calling a lamp is actually a luminaire.