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).
- 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:
- Lojban. It’s been fun and instructive.
- Klingon. It’s also been a lot of fun. Plus, of course, Nick Nicholas’ answer to Have you already had your “15 minutes of fame?” and if yes, would you tell us what was it?
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!