The quickest way I can think of is to give the guest account admin rights, log in, change language, log out, put guest restrictions back.
There must be a better way, but, I can't think of one off the top of my head without being on a domain and using group policy.
edit... Just tried... If you are using a version of Windows with Group Policy Editor (Windows 7 Professional/Ultimate/Enterprise) follow this:
Click the Start Orb and type MMC. When MMC is up, click File > Add / Remove Snap-in, Choose Group Policy Object Editor. Click Browse then click the Users Tab and choose Guest.
From here, Expand User Configuration > Administrative Templates > Control Panel > Regional and Language and you can unrestrict these options for the guest account. Also take a look at User Configuration > Administrative Templates >System > Locale Services.
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: Disc\sources\lang.ini
The contents of my file is:
[Available UI Languages]
en-US = 3
[Fallback Languages]
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.
Best Answer
Do you have Windows 7 Ultimate ? If not that is the problem, cause only in Windows 7 Ultimate or Enterprise you can change from Control Panel the display language. In other Windows 7 versions you will need to hack the registry.
If you are running Windows 7 Ultimate but can't see the options under Control Panel try first to install MUI (Multilingual User Interface).
To change the display language, follow these steps: