You can change two different language settings here at WordPress.com; your blog’s language, and the interface language.
The Blog language is the language in which you write your blog. Each blog has its own language setting, so if you have multiple blogs associated with your account, each one can have a different language setting.
This setting can be accessed under the Settings –> General menu in the blog’s dashboard.
The Interface Language is the language used by your dashboard elements (or user interface). This is specific to each WordPress.com account. It does not differ between blogs.
This setting can be accessed under Settings –> Account Options, or from the Users –> Personal Settings menu in the blog’s dashboard.