Ah, the question asks that I do it for only practical reasons?
Erk. That’s… regrettable. I’ll have to jettison languages I know (Lojban, Klingon) and would like to know in theory (Irish).
The first six, I know. (Well, kinda, as Clarissa Lohr and Kat Rectenwald can attest.)
- English. Because I am Australian, and because English, for better or worse, is the current lingua franca
- Greek. Because I am Greek. What a horrible thing it would be, for me to have restricted access to Greek culture. Or my relatives.
- German. Because of German-language scholarship, especially in my original field of historical linguistics. (And because of the heights of German culture.)
- French. Because of French-language scholarship, especially in my original field of historical linguistics. (And because of the culture, although I’m much more of a Germanophile.)
- Esperanto. Because I’ve learned a lot about other cultures and literatures through Esperanto (including Esperanto culture and literature).
- Latin. For vocabulary, literature, scholarship (yes, you heard me right, scholarship, in the Classics) and all-round cultural foundations of the West.
The next three, I only know bits of.
- Italian. Some literature, and it proved handy to know a smattering of it while vacationing in Italy.
- Russian. There’s scholarship and literature I’d have liked to have accessed, and Russians I’d have liked to have been able to talk to in their language.
- Turkish. For neighbourliness and linguistics.
The last three, I don’t really know at all.
- Albanian. For neighbourliness and linguistics.
- Armenian. To impress my wife. Hypothetically, because actually she doesn’t know much Armenian.
- Mandarin. Because China matters more and more and more in my part of the world.
Now, if fun were a criterion for the language learning (and that’s the criterion most answers have actually applied), the next six are instead:
The two other conlangs I already know:
The two other conlangs I’d like to be across more:
- Interlingua. Just like Latin, only artificial.
- Interglosa. Best designed conlang ever.
The two weird and wonderful natural languages with cultural resonance in the Anglosphere:
- Irish. How cool would that be.
- Old English. Man, I’d love to be able to contribute to se Englisc Wikipǣdia!