Thanks Cem and Baloo,

I'm kinda happy to know that I'm not the only person facing these problems. :D I'll take a look of your solution shortly, Cem. 

Baloo, I agree - even though I have quite much experience, the process was quite slow. The instructions were detailed, but still, this is a quite basic functionality, that should be done by clicking a button.

I thought that I managed to make a copy of english language module (and my own module even appeared in Apps market, but clicking the install button just gives the "Cannot install module from directory"-error. Without any further information, I didn't have time to start researching, what could cause the problem.