Use the Windows 8 Language Profile to pick YOUR language(s)
One of the best new globalization features of Windows 8 is the new Language Profile. From the settings charm, pick "change pc settings", then in the general group, pick "language preferences" to get to the language preferences dialog.
The language preferences lets you pick a list of languages that you're interested in. Like maybe you speak German and English. You can pick "Add a language" and add a new language. Applications will then know that you speak either, and can load resources appropriately. You can prioritize the order of your preference by moving one language above the other, so German then English would show me German stuff if the app had German resources, with English as a second choice.
For some, like German, you also have the option of downloading and installing a Windows display language, and then that language can be enabled for Windows itself. This is also where you can adjust your keyboards and keyboard layouts.
But, wait, there's more! The language profile isn't restricted to system UI languages. You can add other languages to the list, such as Hawaiian. Then, if your application supports Hawaiian, you'll see Hawaiian resources in your app (assuming Hawaiian's on the top of your profile.)
In the Language panel, adding a language lets you search for the most common ones, but you can add any language that has an ISO code, over 6000 languages known by Ethnologue, even Klingon (my favorite test case). For rarer languages, you have to type the ISO code and probably a script code in the search dialog. So tlh-Latn for Klingon transliterated to Latin, or tlh-Qaak using the "Klingon pIqaD" script (Qaak is one of the private use codes since ISO doesn't have a script code for pIqaD).
I love the language profile because it gets rid of those annoying "what language" dialogs when you install apps, because the system already knows your language preferences, even languages that don't happen to match the system UI language. It also means that some apps will "light up" when they support interesting languages that the previous systems didn't know about.
For the developer, their piece of making this all work is ensuring that their application has it's resources tagged properly. Apps in the store have to support at least one language that the store supports, but you can have other languages in your app.
So, users: please use your language profile, that'll give you the best experience for your applications, and developers: please tag your resources properly so that the system can find the best match for the user.