Use GCFScape and open up the Portal 2 content file, found at \steamapps\common\portal 2\portal2\pak01_dir.vpk (for some reason it's unlike TF2 and HL2 which use .gcf's)
There will be a directory structure visible in the program, browse to \root\sound\vo\ and go nuts with probably close to 1000 audio files in a couple folders:
\root
\sound
\vo
\announcer
\cavejohnson
\core01
\core02
\core03
\glados
\turret_defective
\wheatley
Cores 1, 2, and 3 are the...
"space", "fact" and "adventure" spheres, respectively.
The final JoCo song is \root\sound\music\portal2_want_you_gone.wav,
...and some other talking stuff (turrets mostly) are in the \root\sound\npc\ folder.
Outside of the .vpk with the audio files, the subtitle file can be found at \steamapps\common\portal 2\portal2\resource\subtitles_english.txt (or _czech, _danish...) You can actually use this to quickly find an audio file by searching for a word or sentence fragment, e.g. "nine":
"announcer.openingcourtesy01" "<clr:250,231,181>Announcer: Good morning. You have been in suspension for nine nine nine... nine nine ni- This courtesy call is to inform you that all test subjects should immediately vacate [FADES OUT]"
The related audio file would be in the .vpk, \vo\announcer\openingcourtsey01.wav
Best Answer
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,