Section 6.5 of the Windows Store Certification Requirements (v4.0) reads as follows:
You must localize your app for all languages that it supports
Your app can only support Windows Store allowable languages, and at least one of those languages must be a certification language. The experience provided by an app must be reasonably similar in all languages that it supports.
You must provide a complete description of your app for your customers in each language that you declare in your app’s package. The other elements of your app’s description, such as screenshots, text and promotional images, must be localized. If your app is localized such that some features are not available in a localized version, you must clearly state or display the limits of localization in the app description.
It’s important in this context to follow the links to the Choosing your languages topic and clearly understand what is an allowable language and what is a certification language. To quote that topic:
Certification languages—the languages that we support when confirming that your app meets our certification requirements. Your app must support at least one of these languages.
Allowable languages—languages that we support but do not use as part of the certification process. Your app can support as many of these languages as you like, as long as it supports at least one of the certification languages.
What this means is that if you clearly support one certification language (and there are many), you can then check off any number of the allowable language when you submit your app. If you want to check more than one certification language, on the other hand, then you must make sure that your app is localized for that language and you also provide localized descriptions and screen shots when you submit. This is all so the Store can show that app in a regional market with all that language support in place.
In short, be very cautious about checking certification language when you submit, making sure that you’re done all the necessary localization work and testing in your app for that language. It’s better to make your first submission with one or just a few languages at first, then add updates later, than to be over-ambitious and fail certification in your major markets because you neglected a detail for a secondary one.