[Luis]

lol
Then it depends on your specific interest, the other person, the culture...
It also depends on whether the other person is easy going or not.
If it's just for laughs, the other person knows it, and you think you may have fun together, just ask for it. Or if he/she tells you something that is happening to him/her, you may also ask for meeting him/her in a quite place, such that you both talk about that.
But first of all, both of you must have some connection. It's not like meeting a person in the street, and then just saying: 'Perhaps we could meet sometimes!' (though, now that I'm thinking about it... I've made it! O_o But in Germany, and the situation is completely different.)
I think I'm going nowhere with my explanation... XD I'm not sure what do you want me to say reason XD
I'll try it with an example.
I was in Germany, in a disco, waiting in the queue for the checkroom. And I was talking with a Spanish friend of mine (in Spanish, obviously). Then we accidentally pushed the girl behind us, and we said sorry. Then, she 'tried' ('cuz she was german) to say 'Don't worry' in Spanish. Then we started talking to her in German. I ended talking to her in German. After some of her 'Spanish exhibitions' then I suggested her to be m4h Tandem partner. (Which means, we talk for a while in German, so I can learn German and she corrects me, and then in Spanish.) She agree'd, and then she gave me her cell number. I told her I'd call her.
I never did it XD But only 'cuz I was really busy by then! =\

Here in Spain, I used to ask to m4h classmates their number, then sometimes I sent a SMS, just to ask how is everything going. And in time, that makes a connection which gives you a 'reason/excuse' to ask him/her to meet for a while.