The French use the vigesimal system (base-20). I learned this fact while learning Norwegian, which doesn't use a base-20 system anymore, expect almost everybody above 50y still use the Danish system (also base-20, like French), which was indeed very confusing.
In Spanish it's more or less the same as French when it comes to naming numbers, except more predictable spelling and it's base-10: dos (two), doce (twelve), ocho (eight), dieciocho (eighteen), ochenta (eighty), noventa (nighty), cien (one hundred) and so on.
In the case of e.g dos (two) and veinte (twenty), were there doesn't seem to be any relationship, the root of veinte is "dwi" (dos), but it lost a sound somewhere along the way. Two, dos, duo, dual, double, all share the same indo-european root.