Short answer: the language files are not on the DVD, you need to use Windows Update.
The first thing to mention is that it is only possible to change the display language in Ultimate or Enterprise.
I've had a look at my Windows 7 (Home Premium, but they all have the same files I understand) DVD, and it only seems to contain 'en-US' is available.
You can confirm the languages available on your DVD by checking the following file:
The contents of my file is:
[Available UI Languages]
en-US = 3
en-US = en-us
You can also look for language files by browsing
disc\sources\install.wim in a program like 7-zip. Under the Windows folder, the only language folder visible to me is 'en-US'. I compare this to my actual Windows folder, where I have installed Japanese and German language packs, and there I can also see 'ja-JP' and 'de-DE' folders.
So I think the answer is: you can't install language packs from a retail DVD. Googling shows that Microsoft have produced a DVD with all the language packs on for MSDN subscribers, but it's not available for general download.