TL;DR: Almost every category is fastest in Spanish. A few are fastest in French. NG+ is fastest in German.

There are a few points in the game where playing on a different audio language results in dialog ending faster, and allowing you to progress through the game faster. You can view a full spreadsheet detailing all known language differences here.

Fastest Language by Category/Platform

Any%
PC/PS3/Xbox360 - Spanish (9 seconds faster - 7.9 seconds w/Cat)
PS4 - Spanish (6.7 seconds)
XB1 - Spanish (6.8 seconds)

Glitchless
PC/PS3/Xbox360 - Spanish (20.5 seconds - 19.5 seconds w/Cat)
Return to Arkham - Spanish (18.3 seconds)

New Game Plus - German (0.4 seconds)
New Game Plus (w/Cat) - Spanish (2 seconds)

HQR - French (8.4 seconds)

100% - Spanish (17 seconds - 16 seconds w/Cat)
100% (NG+) - French (21.9 seconds - 20.7 seconds w/Cat)

How to Change Language (PC)

Navigate to the game's configuration folder in:

Documents\WB Games\Batman Arkham City GOTY\BmGame\Config\

and open "UserEngine.ini" in a text editor. In that file, find the line (should be line 31) that says

Language=Int

and change "Int" to the language code you want. For taking advantage of the language differences, you must choose a setting that results in non-English audio, interface language only makes one minor difference. The five audio languages available are English (Int), German (Deu), Spanish (Esn), French (Fra) and Italian (Ita).

Here are all known language codes:
Int = "International", Full English. Unrecognized language codes will default to this.
Deu = Full German.
Esm = Spanish interface, English audio.
Esn = Full Spanish.
Fra = Full French.
Ita = Full Italian.
Jpn = Japanese interface (at least in theory, the text doesn't work for me), English audio.
Kor = Korean interface (in theory, same deal as Jpn), English audio.
Pol = Polish interface, English audio.
Por = Portuguese interface, English audio.
Rus = Russian interface, English audio.