While it is undoubtedly true that every speaker of a language possesses unique traits of speech production that constitute what is called an idiolect, it is nonetheless also true that native speakers of any language adhere to certain statistical norms in producing speech that are characteristic of that language. These norms are what make it possible for speakers to identify linguistic tokens of a given language as authentically English, German, Russian, etc., although they may not be able to state what these norms are. It is also what enables speakers to make correct judgments about speech that deviates from authentic instances of native speech.
The intuitive grasp of statistical norms of speech production is illustrated by the following occurrence on the streets of New York, where over eight hundred languages are purportedly spoken at the present time. YHB was walking west on East 71st Street in Manhattan a few yards behind a woman pushing a stroller, close enough to hear her speaking on a cell phone without being able to tell what she was saying. To a native speaker of Russian able only to recognize the intonation and the general phonetic profile of speech being produced “into the air,” it was still possible to make an educated guess that the woman with the stroller was speaking Russian. This guess was indeed confirmed when the distance between speaker and hearer became narrow enough for the language to be recognized.
In the same way, every person with a sufficient command of a language (and not just native speakers) can identify a foreign accent, although the ability to “place” the accent varies with individual linguistic acuity and experience. Some foreign accents are so common and so broad as to be routinely identifiable without difficulty. These are the accents that commonly lend themselves to mimicry and to theatrical imitation for comic or parodic effect.