Steam game audio doesn't change to Traditional Chinese

I have tried changing the game audio settings of Portal to Traditional Chinese with no luck. Only subtitles seem to change, and that is after changing the language settings of the Steam client. The game is clearly marked as having full audio support for Traditional Chinese, and there are proper menus in-game and in the Steam Library to set the game audio to Traditional Chinese.

Setting the game audio to French and German works (English too, perhaps obviously). These languages all start a download of about 100Mb. There is no download when setting to Traditional Chinese.

Is there anyone who can make these settings successfully, or who knows what I am doing wrong or what I should do to solve my problem?

Edit

I mentioned this to Steam Support, who provided me with the below answer (which did not solve my problem):

Thank you for contacting Steam Support.

We apologize for the delay.

Please exit Steam and go to the folder called C:\Program Files\Steam\ (this is the default location for a Steam installation - if you set a different installation directory, you will need to browse to it).

Delete all of the files in this folder except:

• Steamapps and Userdata folders

• Steam.exe (this file is listed as an application and features the black and white Steam logo)

Restart your computer.

Then, launch Steam.exe from within the Steam installation folder, and not from a pre-existing shortcut.

With Steam running, re-test the original issue.

Note: This process will not affect your currently installed games.

If the issue persists, make sure your network is optimized for Steam by following the guide below:

Title: Troubleshooting Network Connectivity Link: http://support.steampowered.com/kb_article.php?ref=1456-EUDN-2493

I am currently awaiting further instructions.

According to this steam forum post, this one too and this third one, Portal doesn't have full audio support for Traditional Chinese (note the lack of star beside Traditional Chinese in the list below) despite the home page saying so. Either it is missing as the last post says or it's a mistake on the steam page.

A few languages has no audio support

Languages: English*, French*, German*, Russian*, Danish, Dutch, Finnish, Italian, Japanese, Korean, Norwegian, Polish, Portuguese, Simplified Chinese, Spanish*, Swedish, Traditional Chinese

*languages with full audio support