Hmm, Frisian is a language of itself, more closely related to old English than to the other Germanic languages. There are only a few small remaining groups of people which speak the Frisian language, which now consists of about 3 different major dialect groups, which can then be subdivided even further. The west Frisian (Frysk) dialect located in Eastern Netherlands of course has the most speakers. The dialects spoken in Germany of Frisian are often considered by many Germans (to my surprise) to be German dialects, although this is of course not true.
In regards to Dutch, German, Plattdeutsch and English. These languages are all closely related, although some to a greater extent than others. German and Dutch are very closely related, although it seems as though many people do not want to recognize the similarities due to political reasons, always looking for the differences. I have heard various things from a large number of people and sources, but the general tendancy seems to be, that Dutch and German are NOT mutually intelligable languages, although can be understood to a small extent. I have talked with several Dutch people which claim to be able to understand varying amounts of German, however most Germans that I speak with state that they cannot understand spoken Dutch but can read bits and pieces. An online source had stated that German and Dutch share maybe 80% of the vocabulary, although this souce was not scholarly and should be taken, only for what it is worth.
I am a native English speaker, although I have studied German for 3 years. I have a love for a few different Dutch and Belgian musicians (that sing in English, however), and this sometimes browse through the internet forums and see Dutch here and there. If there is a simple message being conveyed, usually I can understand it fairly well without consulting my dictionary. The Dutch grammar is also more similiar to German than to modern English, when one considers the verb placeage, verb conjugations, etc, although the Dutch grammar is more simplified than the German grammar. In regards to spoken Dutch, I rarely have the chance to listen to Dutch spoken, but I also have a few Dutch musicians which sing in Dutch (Marco Borsato, Maud), and I can understand more than one would expect. However, these are just my observations and I suspect that some of the largest differences are mainly political, although I do not want to step on anyone's toes with my observations...
I also know that there are some dialects of German and Dutch which lie between these two languages. Basically, what we have is a language/dialect continuum, and there are dialects of German which are spoken near the border of Germany/The Netherlands, and the same with Dutch. These dialects may share parts of both Dutch and German...kind of lying in between the two languages, with characteristics of both. Once again, it depends on which side of the border the dialect is spoken, as to whether it is classified as a Dutch or German dialect.
Plattdeutsch is 'Low German,' and is spoken in northern Germany...many of the words are more like Dutch or English, since they did not undergo the second gradual soundshift which the southern 'High German' dialects underwent.
I think many Americans think that Dutch and German are the same, because they do share a lot of vocabulary and grammar, but also because we have the Pennsylvania Dutch, which are actually a group of German-dialect-speaking people.
I am not a linguist, although a bit of a closet linguist you could say. Hopefully this information helps clear some confusions up. I am also studying Dutch to some extent, although the pronunciations of such sounds as 'ui', 'g', and 'ch' are always tripping me up
