I want to develop a commercial game.I hope to develop this game until 2012 December,but I'm having a lot of problems to chose what tool I need to use to develop this game.I want to release the game for Windows,Mac and Linux(maybe mobile or web too but that's not what I want by now) so in which tool I will have less problems to learn and develop a game in cross platforms?
Python-Pygame?
C#-XNA?
Action Script-Flash?
Js-HTML5?

Ps.:I've made some games(in fact some demos) on Unity so I'm not a completely noob I didn't chose unity for this project because I want to develop a 2d rpg game and unity isn't good to develop a game like this(no tilemap editor)
Ps².:Sorry for my bad English
Ps³.:Sorry if my questions are stupid

1) There is no such thing as a stupid question.

2) If you want to release the game cross platform, choose a language that has graphical libraries which are cross platform. All the languages you suggested have them, however, you also mentioned wanting a mobile and/or web/browser version of your game. For this I would suggest Js/HTML5. If you just want to stick to PC/Mac/Linux I'd go with either C#/XNA or python/pygame.

That said, it's really up to you, and my response is based off very limited information (I have no idea what your plans are for this RPG or what you're trying to achieve so I can't give you any advice in that respect).

I have no experience with Pygame, but I can recommend XNA and Flash. I would not recommend html 5 in its current state for anything more than experimenting with html5. Some might disagree with that, but can't get myself all the way behind it atm.

If this is single player, then Flash might not be a good choice. It's better for free game that make advertising money. Also, there is almost no money in the Linux market. Your best markets for commercial games are windows and iPhone. If you are going commercial, then think about profits. On the pc, your best retailers are Steam, BigFish, and maybe Popcap, so do some research on what type of game does best and any requirements. C# XNA probably has the most possibilities for different retailers. You can go XBLIG, Steam, BigFish and probably Popcap. The only thing you couldn't do is iPhone.