I've been mad-crazy about languages since I was very young. I started inventing languages when I was in about the 5th grade, though those early attempts were very crude. I based one on Latin that was a mess.
When I finally got to college I went nuts: Classical Chinese, Japanese, Sanskrit, Ancient Egyptian, Greek and Latin. I've taught myself Old English, Coptic, Hittite, Biblical Hebrew and little bits of an army of others. Notice the emphasis on dead languages? I have a reading knowledge of these... I can't speak a one, and I need a dictionary handy. I can get by in modern Chinese and maybe French if pressed.
One result of all of this language studying, especially the Latin and Greek, is that I do immoral things to English grammar and I have an overactive vocabulary. I love old-fashioned ways of writing with lots of parenthetical statements and huge sentences, to the horror of all my English teachers. They've mostly rid me of this baroque writing style, but it creeps into the light from time to time.
I'm a firm believer in learning a classical language. Learn several, ideally. Before I get accused of being Eurocentric I consider all the following classical languages: Greek and Latin of course, Classical Arabic, Sanskrit and Classical (literary) Chinese.
Anyway, here are the links:
Langue d'Oc This is the language of the Troubadors. Not quite a classical language, but important for the cultural history of Europe nonetheless. Dante almost wrote the Divine Comedy in this, and for some reason while the rest of Europe was still using Latin in official and literary documents, those living in what is now southern France were developing their own colloquial language into quite a subtle and sophisticated instrument. Note that language of the Troubadors still lives (though barely) in France, and is best known as Occitan, and the region Occitania. The language is sometimes also called Provençal.
Some other languages