I would say don't. Unless you are in Wales or France. (They are not the same name, just spelled the same). My dd is a Manon, we are a Welsh family in a very welsh speaking part of Wales and it isn't an uncommon name here. However many of my English friends call her by the French version, or as a pp said they shorten the O so that she is more Man'n. DH's cousin also comments that "Man on" is a football thing? So unless you live somewhere where it will be pronounced properly then choose something else would be my advice!