Language and Dialect
Linguistically, mutual intelligibility is often described as an important factor for the definition of a separate, independent language. Accordingly, native speakers of one language are usually able to understand each other. Between native speakers of two separate languages, such as English and German, mutual intelligibility, however, does not necessarily exist (except where multilingual competence is given). Thus, we can say that the linguistic varieties A and B are two independent languages if the native speakers of A are not able to comprehend the native speakers of B and vice versa.
In this connection, a language can be seen as a continuum of linguistic varieties, i.e. dialects. That is, it has a certain spectrum or range characterized by different dialects and accents. Accordingly, British English is one language which includes different local varieties (dialects). The same is true for American English where we find regionally differing language varieties. Of course, speakers of British and American English are able to understand each other. Likewise, German used in Germany or Austria is one language with different regional varieties. Thus, a language is a collection of dialects or linguistic varieties. Native speakers normally comprehend the dialects of their mother tongue.
We may say that if two dialects share a common grammar and vocabulary and are mutually intelligible, they may be treated as belonging to the same language. However, as mutual intelligibility may be a problem in practice, this is said to be only a general rule of thumb.
Click here for an illustration and distribution of dialects in England and the U.S.A..