There are many German dialects, as can be seen on this map, and these dialects can be a problem even to native German speakers. It is estimated that there are between 50 and 250 German dialects, so if you peak standard German, you might find yourself lost when the locals talk among themselves.
It is especially problematic in more remote and rural areas, where regional dialects are more likely to be spoken. Here schoolteachers who have moved from other regions sometimes can’t understand their pupils at all. Even TV documentaries sometimes have Standard German subtitles or voice-over because the people featured in them speak a dialect that is incomprehensible to the majority of viewers.
One interesting note: the historic Angles and Saxons who migrated to Britain were from regions in northern Germany, where you still hear the somewhat medieval Plattdeutsch, or “low German”. The Anglo-Saxon period of rule in medieval Britain started at the end of Roman rule….an era known in European history as the Migration Period of 400 to 800 A.D.(the Völkerwanderung or “migration of peoples”). The old north German Plattdeutsch thus became what is known as “Old English”… the early form of the English language that was spoken in Anglo-Saxon England and southern and eastern Scotland from the mid-5th century and the mid-12th century.This is why low-German Plattdeutsch in Hamburg, for example, shares more words and similar phonetics with English than with modern Standard German.