I am a former language student (mainly romance and slavic languages), so you can send the card in the language of your choice. :D
For the cards, it's hard to define what to send, even more that I change my opinion regularly. But I can define a few ideas, not as a wishlist, but rather as a source of inspiration (given possibilities of course, I know how difficult it can be, to find different cards, depending on the cards sold in your area):
At the moment, I am suddenly rather fond of train cards, mainly old trains and locomotives. Or trains in impressive surroundings.
And for postcrossers from the USA, I just want to see if I can get cards from all state capitals. Just a nice one with a view and the name of the state/capital on it. :)
Another topic could be cinema, actors, movie makers (mainly if somehow related to Kaurismäki, to Monty Python, Fellini, Pasolini, Andrei Tarkovsky, Jim Jarmush or others) or music. (I wouldn't dare to start a list of examples here, it would be far to long. :D Unless you have a card concerning Finnish Tango, Ismo Alanko or Tom Waits, that would be great. But pretty please, no Musikantenstadl card.)
If you prefer to send a scenery card, I don't mind that much where it is from. Of course, it would be better if it is from your area, or country, or continent or planet, but the main thing, is that it has an interesting picture. Animal cards are welcome too, though I would prefer "artistical" ones... or funny one. :) Just not overly kitsch, if possible.
I received quite many illustration cards lately. I still like them, but I just wanted to put this a little lower here. But if you have cards from comic books (as far such cards still exist) from Gotlib, Alain Ayroles, Gerhard Seyfried, Ralf König (or similar artists), I might be really happy.
If you have a special/strange/funny/bizarre/weird card you are not sure whom to send, don't hesitate.
Concerning ad-cards, they are not my first choice, but there seem to be some very intriguing ones out there, so no worry with such cards either if you think they match my profile.
In short, all cards are welcome.
In case of doubt, pick 6 random cards, place them in line, and send the third card from the left. :) (After several cards received that way, I can only say that the system works very well.)
Or eventually check my favorites list, it might give some hint. (Unless it confuses you even more - but then it might be interesting, what you conclusion is. :D)
And please (most users do it anyway, but in case of), send the card with a little text and stamped with no envelope. If possible of course. (Don't worry too much for the text, just write what crosses your mind, be it something about you, about your town, the weather, the last thing that made you smile, I nice joke you heared, your favorite word, if klingon should be mandatory in school.)