Tanzie I am convinced it's down to verbal skills she picked up as a very young child. And I can bet you if she was to try Romanian again (i.e go to that country or live with a native family for a short while) it would all either come back to her (the human mind probably has this ability) or she would learn it as a new language but would 'know' how to pronounce words.
As French is probably similar with Romanian (perhaps some sounds, particulalrly consonants, are stronger) maybe that's why she can pick it up so easily. Interesting to look at how much French influence there is in the Romanian language too (like there are lots of imported French words in Britain).
If you know of other languages that are similar to Romanian, you could try introducing them to her as an exercise and see if indeed she shows an aptitude for learning them particularly easily. You could also try languages which are totally different like Japanese (I can safely assume this has nothing to do with Romanian) and see how that goes. I'm only talking about a few basic words not a whole new language.
I imagine there must be at least 3 different important aspects here:
1)what I've said before about 'training' specific muscles used for speach and developing the ones specific for the chosen language at a very early stage in life. From what I gather, it's not a process that can be picked-up later in life, probably the process stagnates after a certain age.
2)The human brain is a fantastic machine. It may well stock up information , lots of it, from very early on, which somehow can be released at a later date if triggered by the right factors (whatever those might be, none knows)
3)The neurons (brain cells) are only formed up to a certain age (don't know which one exactly but fairly young). Neuron connections are formed when we learn something new and by exercising that knowledge we maintain these connections in a functioning state. Most of the cells in our body renew themselves because this is how our body works. neurons are the only cells never to renew themselves. However, it is vital that they have rest periods which can only be provided by resting (that's why it is vital we sleep, presumably meditation at its highest level works the same way). So to my mind, if there were some neuron connections formed as a result of aquiring a new language, hopefully they still exist later in life (unless damaged by some brain malfunction) and hopefully they have survided despite not being directly 'exercised' through the use of that language because the child stopped using it or maybe these connections can be used for different tasks in paralel.
No idea if any of this makes any sense to you or if indeed is acurate. All based on what I've read/learned/heard over the years and expressed in a very simplistic way.
Anyway, to sum it all up, there must be some relation between learning a new language early on even if stopped using it and learning other languages later on in life.
Also, maybe having a musical ear has something to do with it in terms of accuratelly replicating sounds that make up words in a foreign language.
All very interesting stuff, shame we don't know how it all works!