Is it possible to have multiple cultures supported in a WP7 application, but with a unique language description in marketplace? I can maybe translate some keywords, but writing a description in any languages is not easy!
You'll have to write a description, in each supported language, for your application. So if you want to support multiple cultures, you should ask people on the internet to help you translate the descriptions.
You can't have a single, English, description of your application, that's used for all cultures.
The marketplace will detect what cultures you're supporting in your actual application, upon submission.