I'm from Czech Republic and I love playing Portal 2. I'm quite good an English speaker, but I just don't understand some parts of what they're saying. I was looking around the internet. But I can't find anything. Save me, please. Any ideas?
Portal 2 does not have Czech dubbing.
If you want to change all of he game's menus and interfaces into Czech, change your Steam language to Czech. This can be done in Steam interface by going to Steam/Settings/Interface/Select the language you wish Steam to use (requires Steam to restart)/Czech and restart Steam.
If you just want to change the closed captions, go to the developer console and type cc_lang "czech". Note that closed captions must be enabled from within the game under Options/Audio/Closed Captions for closed captioning to be visible. By default, if your Steam language is set to Czech, you should not need to get to the developer console, but closed captions would still need to be turned on.
All this information can be found on the Steam forums.
And in case you were not aware the wiki describes how to access the developer console,
You can enabled subtitles under Options -> Audio -> Closed captioning. "Full Captions" means every sound will be subtitled (e.g. "Door opens", "Turret activates", etc.) and "Subtitles only" means only speech will be subtitled.