The language should you use when writing a blog. There are a number of languages supported by Google AdSense program, eg: Italian language, mandarin, indonesia, latin and many more.
If you use other languages that are not supported by the AdSense program, even though you can insert Adsense code, but sometimes ads will not bubble up by Google for an unsupported language. Do not ever apply for a Google AdSense account with a blog with an unsupported language. Google employees will not understand, and your application will be rejected.
The language used