I’m a woman reader of O’Brian. (Left Hand of Dorkness is not a reader of O’Brian at all.) For the most part, Diana annoys the shit out of me. She’s one of my least favorite characters in the series.
I’m a female reader, and I rather like Diana. She is really a modern woman trapped in the nineteenth century. She wants to live the free and easy life of a man, but is constrained by society’s view of what a woman has to be. Her seemingly cruel and capricious treatment of Stephen and other men is the collateral damage of her struggle to find freedom.
Also, I believe Patrick O’Brien saw the character of Diana as representative of his own wife:
After O’Brien’s wife died, he killed off Diana in the series.
Dammit. Go to link to discuss Dianna.
Speak for yourself.
Brilliant physician and naturalist, international spy and diplomat, speaks 6 or 7 languages (at least), competent with a cello, deadly with both pistols and a sword, married to a beautiful woman way out of his league, had stones enough to operate on himself and remove a musket ball from his own chest. All while ingesting massive amounts of laudanum and/or coca.
Maturin rocks.
Thinking about it further, I can come up with at least 10: English, French, Spanish (Castilian), Catalan, Irish/Gaelic, Latin, Greek, Hebrew, Arabic, and Hindi/Urdu.
I did say, “pale version”. 
Didn’t he take an assistant who spoke Arabic? A jewel dealer, as I recall.